I heard somewhere once, I think it was on Skeptoid, about how a conspiracy theorist's brain functions on a more primal kind of instinct. Most people evolved to not be nervous about the world around them, because we are not longer generally the victims of predators and what not. So the modern extension of the primal instinct is to be concerned that everyone, the government, authority, etc., is out to get you or whatever. And that kind of specific aggressive lack of trust takes you down the path of not believing any established normal explanation of things.

I bet that post WWII and in the 50s, if people were polled, there were less conspiracy theorists than in the post-Vietnam era, since that seems like when the general public's trust in government, leaders, etc., really began to evaporate. Maybe not, but seems like there would be. In the 50s and leading up to Vietnam, I think people were more concerned about other countries/communism/etc as the threat, and less so their own leaders.