Because certain factions within the government don't want the truth to get out, and have spent decades convincing the public that it is akin to some fringe conspiracy theory (helped out by a lot of legitimate conspiracy theorists) so that the leaks that inevitably do happen are dismissed out right. However, if you do actually spend a bit of time looking into the issue,there are actually loads of credible folks involved who have tried to get the truth out. So now we are left with a situation where a significant group of otherwise rational people can't take a rational look at the evidence and come up with their own conclusions due to preconceived biases based on decades long misinformation campaigns.
Now we have a former intelligence officer who's job it was to investigate this stuff blowing the whistle on governernment programs with no oversight. A whistleblower who has bipartisan backing that is unheard of in this political environment. And some of the congress critters fighting for disclosure are undeniably passionate about their beliefs, whether or not you agree with them. Take AOC, I'm sure that many folks here don't agree with her vision of how we ought to be doing things, but it is hard to deny that she has an honest passion for serving her constituents and doing what she thinks is right. Yet she is one of the ones leading the charge.