It's collective behavior. A transitory and passing behavior by people. They just tend not to check the validity of the story and believe it blindly. You know the story of War of the World? It is aired in USA many years ago on radio and during the time it was aired, some even took it as a real story go went to the extent of phoning their friends and relatives by calling them to flee or saying the final words. It sounds funny but it shows that humans are very irrational when subjected to any collective behavior.

I'm going to make a generalization is that less educated are more prone to believing in conspiracy theory as compared to more educated folks.