In the example of 9/11, there were MOUNTAINS of video footage, news articles, and documents stored in a large array of community archives that started as community efforts to find Osama Bin Laden and as it dawned on all of us, slowly morphed into trying to put together a solid fully documented and supported narrative of what actually occurred on that day.
Community leaders disappeared under mysterious circumstances and then all of those digital archives disappeared.
People who were in those communities and helped to compile those archives saw it with their own 2 eyes and remember what was discovered.
People who were not in those communities don't believe they ever existed.
Edit: Looking at these comments, its just precious.
Question "If the government is hiding information, how did you guys have this information."
"I am older than you, or was more aware than you, and I saw the information before the government hid it and turned a historical record into a conspiracy theory."
"ANOTHER PERSON CONVENIENTLY SAYING THERE WAS INFORMATION AND NOW THERE ISN'T!!! DON'T LISTEN!!!"
Basically the prompt is "explain to me what makes a conspiracy theory", and the comments are saying "IT'S A CONSPIRACY THEORY!!!" in response to people sharing what makes a conspiracy theory...