Argthtjtr

I think in Stargate the general population of Earth never knew that Stargates and aliens existed.

So I get that the idea behind the show is that there is a secret operation that most humans don't know about. And, at the beginning, it makes some sense.

Although I haven't seen all the episodes I remember several where many earthlings actually know about the "truth". With all the other outposts, teams, star ships, etc. and incidents happening, there must be thousands of people who know. It seems highly unlikely that this information doesn't leak.
There's even a story arc about leaking information.

So, because it is one idea of the writers, the characters think they shouldn't tell Earth. I get that.

But why did the writers think so? Is there any reason for this?

They let them visit many planets where everyone knows and nothing bad comes out of it. And it seems later in the show, they try to wind around the fact that it doesn't make much sense to not tell humanity and it actually seems that half of them know already.

So why not just let Earth know? I even think this would have created great new possibilities for the show. Like how does Earth cope with knowing, civilian exploration teams, aliens on Earth interacting with people, and so on.

So why did the writers never consider that?

In short, the idea of the Stargate being revealed to the world and the consequences thereof (shock!, horror!, wonderment!, amazement!, riots!, new cult religions!, etc etc) was so big a story that the writers were planning to reserve the plotline for a movie that ultimately never happened.

Q. “Did you plan a stargate reveal at some point ? Ever since it was planned to happened at the end of “Lost City” then cancelled when
SG1 was renewed, i’ve always hoped we would see it at some point.”

Joseph Malozzi: This subject (the revelation and its many implications) would have formed the plot for the planned SG-1 movie, Stargate: Revolution.

_{^{Stargate: Universe, Beyond Season 2! What Might Have Been!}}

and

Wright had previously told GateWorld that revealing the Stargate to
the general public would be a major event in the fictional world and
for the franchise — certainly a “revolution” — and was big enough that
he wanted to reserve it for a movie. It was an idea that the writers
enjoyed toying with in the television series, including the
alternate-timeline episode “2010.” In that episode, Earth’s Stargate
was kept in a public “space terminal” and available to the public for
travel to other, allied world

_{^{SG-1 movie would have revealed the Stargate}}

First, there's progressive declassification of the program over time - more and more nations are included on the secret at least; by the time of Atlantis, it's pretty much a worldwide project (though still very secret).

Second, the scenario is explored in a few stories - either as alternate Earth realities (as a result of time travel or just some "parallel universe") or on other planets. Some are happy, some less so. Most of the time, cultures that are still rather primitive or sheltered are fine, while more advanced civilizations don't take the discovery very well. Off the top of my head:

(Needless to say, this contains spoilers)

• Stargate: Continuum - In the alternate reality, Ba'al publicly appears on Earth, and starts diplomatic contacts (interestingly, in this reality the Stargate hasn't been a secret for long - the Stargate program never got started). There's not much time to see people's reaction, since not long after that the Goauld start their bombardment.


• 2010 (SG-1 S4E16) - The alternate reality where Earth was helped by the Aschen, who destroyed the Goaulds which eliminated the original need for secrecy (i.e. telling people there's awful enemies out there which hopelessly outmatch us). The Aschen also required the Stargate be made public. In this version, the transition seemed mostly peaceful, and Earth joins the Aschen confederation with routine travel between planets for all sorts of people.


• Learning Curve (SG-1 S03E05) - Humans taken to a different world a thousand years ago who forget all about the Stargate and fight off Goauld on their own at some point. When Earth makes contact, it seems the locals (Orbanians) didn't even try to keep the Stargate a secret and immediately initiated diplomatic contact and even technology exchange. They seemed to be pretty excited about the possibilities of the Star-gate.


• Past and Present (SG-1 S03E11) - SG-1 comes to a planet with people who lost all their memories. They don't seem to be too concerned about the discovery of the Stargate, though given their memory loss they're probably used to surprises. The civilization seems to be industrial, though somewhat behind Earth.


• New Ground (SG-1 S03E19) - SG-1 comes to a planet where two major nations are in a state of cold war, their differences being mainly religious - one side believes humans came to their planet from another world, the other that they were created in place by their god. When the creationist military finds out about the Stargate, they immediately try to keep it a secret; it's not clear if they simply buried the Stargate or if their cold war turned hot, but SG-1 never regains contact after they escape. Given that one of the sympathetic scientists escapes along with SG-1, it's unlikely they didn't try again.


• Beneath the Surface (SG-1 S04E10) - The government seems to want to keep the Stargate a secret; they seem to be a despotic government that keeps people in line through the threat of an ice age, and the Stargate would probably threaten their power.


• Absolute Power (SG-1 S04E17) - As the US deploy satellites capable of defending against Goauld ships in Earth's orbit, they plan to reveal the Stargate to the public; however, this is interrupted. The main thing to take from this is that the original plan was to make the Stargate public as soon as Earth has credible defenses against the Goauld threat; on most planets we see where knowledge of the Stargate is public, the people are either enslaved by the Goauld or are themselves more powerful than Goauld or protected by someone who is (e.g. the Nox, Asgard, Omayacan...).


• Meridian (SG-1 S04E21) - Three superpowers on the verge of war, one of them (Kelowna) discovers the Stargate when SG-1 comes through; they want to keep the Stargate a secret and get help from Earth.


• Shadow Play (SG-1 S06E07) - The Kelownians from Meridian contact SGC again; SG-1 suggests they tell the other two nations about the Stargate, but the Kelownians are reluctant - they didn't even tell their own people. When O'Neill suggests they make the Stargate public, as the knowledge might unite the three nations, they respond it's not that simple, and that SGC isn't public either.


• Cure (SG-1 S06E10) - SG-1 finds an industrial civilization that just discovered the Stargate, and the Stargate seems to be out in the open, so possibly isn't considered a secret. However, the government also seems to have huge amounts of power over the people, so there's hints about some totalitarian practices (if you stop being useful to the government, they take you of the titular cure and leave you to die).


• Memento (SG-1 S06E20) - SG-1 travels on the Prometheus, when they're forced to jump to a nearby planet with a gate address and dump their power core due to fault in the hyperdrive. The planet turns out to be rather advanced and opens fire. They have no knowledge of the Stargate, and SG-1 asks them for help with finding the Stargate and getting supplies from home. Neither the arrival of aliens nor the discovery of the Stargate seems to be considered something that needs (or can) be kept secret, though there's also an attempt at a coup as a result of SG-1's appearance. So this somewhat shows another fear - if someone unsavory finds powerful alien technology, they might use it to conquer the world.


• Homecoming (SG-1 S0702) - Anubis arrives in orbit of the Kelownian planet Langara from Memento. Hard to keep this a secret. However, we don't see much about the impact of this - the three governments keep bickering as ever, and it's not clear if the knowledge of the Stargate has been made public (though at least they don't have the same reason as the SGC - people already know about dangerous aliens).


• Revisions (SG-1 S07E05) - An advanced civilization is visited by SG-1 and learn about the Stargate; it doesn't seem to make much of an impact. At the end, they're relocated to another planet.


• Lifeboat (SG-1 S07E06) - Investiganting a crashsite of a huge popsicle ship, SG-1 helps save the frozen survivors and relocate them to their target planet. No real impact from learning about the Stargate, though it was something they would have died without, so there wasn't really any alternative but keep it public.


• Space Race (SG-1 S07E08) - An advanced culture with FTL ships we've seen earlier that defeated Goaulds on their own without knowledge of the Stargate was informed about the Stargate. There's some struggle over who gains access to the Stargate, but mostly it seems that the Stargate is made public with little trouble.


• Fallout (SG-1 S07E14) - Again on Langara, a planet-wide threat forces the three governments to consider partial evacuation through the Stargate; it seems that they haven't been using the Stargate in the meantime, since they completely relied on SGC to arrange everything.


• Icon (SG-1 S08E05) - SG-1 arrives on an industrialized planet, where the Stargate is kept in a museum. When it activates, it isn't kept a secret. This results in a surge of religious fanaticism that results in a world war including a full-scale (presumably nuclear) missile exchange a few months later and the government is taken over.


• Covenant (SG-1 S08E08) - A wealthy businessman tries to make his clues about alien life public, and is told about the Stargate in an attempt to shut him up without success. Eventually, he's silenced by the NID.


• Ethon (SG-1 S09E15) - The world from Icon is completely devastated from yet another war, with seemingly no survivors, over yet another religious war.


• The Road Not Taken (SG-1 S10E13) - Carter gets transferred to an alternate reality where the Stargate has been public since the Anubis attack (three years earlier). It's pretty clearly suggested that the US (and perhaps the Earth as a whole, hard to tell) is a dictatorship as a result of martial law enacted after widespread riots at the revelation.

One other reason is that it would have caused in uniervse event what would have been bad for the show out of universe. Example:

One plot ... not hole, per se, but stretch ... is that the main group of four stay together so long. It's highly unlikely that a group of people with as much seniority and expertise would continue to be allowed to go offworld after the first few years. In reality, they'd almost certainly be rotated out and used as training officers for the next group of people. (Especially Carter and Jackson; they are both the top experts in the world in their fields)

It's already a stretch, as is. If the gate were to be revealed, it would cause massive upheaval across the world. One thing that seems almost certain is that leadership would change and evolve. The relatively small group of people who have managed to retain control over it would likely be displaced by a more global and more civilian group. The main four characters would absolutely be pulled out and replaced by new low level people.

That would be very bad for the show, because they are the main characters.