I saw a post in someones blog recently about conspiracy theories and why they seem to be popular. The post essentially said that it's because humans need narrative, and conspiracy theories have the advantage of providing a narrative that's rich and satisfying.

It occured to me on the train last night (a little before others in the carriage realised that Penrith isn't on the Newcastle line) that this is much the same instinct that drove earlier groups to create myths, legends, and god-stories.