Conspiracy theories are a natural result of general skepticism, general ignorance and mass hysteria.
It's natural to doubt, we may be a pretty gullible species at times, but at least some of the time we have a bullshit filter trying to flag stuff up.
The problem is that, on top of that, we're also generally pretty stupid- we don't know much about most things but we like to pretend we do. I have no way to know what happens when something like a plane hits a building, I can't really give an informed opinion on that. So, sometimes our intuition will be plain wrong and we'll get false positives.
Add to that confirmation bias and charismatic people presenting their bad intuitions as 'fact' with hand-picked evidence that supports their point of view, you end up with a vicious cycle of hysteria over something that has no real basis, only a self-sustaining presupposed conclusion.
The problem with most conspiracy theories, in my mind, is usually intent- 'Don't attribute to malice what you can to stupidity'. Big organisations are stupid and slow. They can't do anything that well and it always takes a while to do them. Sometimes they will try to do malicious, cunning and evil things, but anything like that on a big scale there's a large potential for getting found out or slipping up. The more people involved, the more likely it's bull that they pulled off such a wide-reaching conspiracy and nobody said anything about it. Whistle-blowing seems to be cool these days, so you can bet your front teeth that if people could come forward about such things, they would be by now.
