It's hard to believe in big conspiracies requiring many conspirators. It's much easier to believe in small conspiracies requiring only a few conspirators.
When the government controls the agencies doing the investigating, and the conglomerates control the news media doing the reporting, it isn't hard at all. They've officially maintained the coverup on the JFK assassination for the past 40 years. They're still planting TV programs showing cartoons of TWA 800 breaking up due to a center fuel tank explosion.
There were many witnesses who saw a missile but the government and the press called them mistaken or liars when they referred to them at all. Many of the witnesses were told in an intimidating manner that they were mistaken and that was crazy to talk about it at all. It does not need great numbers of people keeping their lips sealed, only the cooperation of the press and the relentless repetition of the official story.