The flagrant stupidity of most conspiracy theories popular during my lifetime, as epitomized by Oliver Stone’s 1991 masterpiece/fiasco JFK(in which the entire military-industrial complex plots to murder John F. Kennedy by hiring some flaming French Quarter homosexuals), serves to inoculate the powerful against the suspicion that they have influence (or responsibility) regarding events. 

It wasn't always like this. Until recently, it was widely understood that numerous turning points in history—such as the assassinations of Julius Caesar, Abraham Lincoln, and Archduke Franz Ferdinand—were the results of conspiracies.