Why conspiracies?
I think William Manchester hit it on the head when he wrote about the wave of conspiracy theories in the Kennedy assassination.
Manchester said that humans need balancing on the scales for events of such titanic nature. The idea that someone as towering as Kennedy should be killed by such a worthless waif as Lee Harvey Oswald doesn't make sense, and does not do justice to Kennedy. We want the villain to be a super-villain, not just some mope, mook, mutt, loser, or jerk, as cops would say.
People also have trouble with ambiguity and open endings. Centuries of moral fables, fairy tales, and soap operas have created a demand in humans that everything be tied up neatly by the last minute of the show. We expect the killer to be the special guest villain. We expect Kojak and Steve McGarrett to snap their fingers and point out the villain. We further expect the villain to break down and confess under the lash of cross-examination by Jack McCoy or Perry Mason. We also expect John Wayne to win all the battles (even if he dies in the final scene) and that a bunch of Japanese, ragheaded Arabs, redcoated English fops, or sneery drug dealers are nowhere near as smart, capable, or clever as our generals, patriots, and detectives.
Conspiracy theories provide neat and pat endings to these disasters. They play into our fears of the unknown, demonize the usual suspects, and tap into the cynicism Americans have festered since Vietnam, Watergate, and all the other idiocies since then. Remember that the United States ground to a halt to investigate Monica Lewinsky's semen-stained dress...while Osama and his pals were scouting targets for their second attack on the World Trade Center. We cannot accept that the other guy hates us enough and is smart enough to wreck our lives...it has to be the result of a hidden and treasonous cabal that is selling out our nation, our values, our rights, our moral fiber, and our heritage.
Conspiracy theories turn a random and ambiguous world into a neat and coherent whole package...it's all the Jews' fault. It's all the Masons' fault. Big Oil did it. The bankers did it. The Communists did it. God did it. It's all in The Bible. All in Das Kapital. All at the Mason's headquarters. All in Christian Rakovsky's confession to the NKVD in 1937.
The world becomes orderly, and those who feel victimized by the world have convenient scapegoats and targets to blame for their own failures. It's not my fault my business failed...Big Oil destroyed me. It's not my fault I couldn't get into a good school...Affirmative Action quotas beat me out. It's not my fault I'm not rich and powerful...the Jews have all the money.
They also enable people to be lazy. What's the point of trying? Elections are rigged. The Jews have all the money. Better you just sit back in your chair and whine about the conspiracy on an internet forum of like-minded losers than actually try to make your life and world better.
Conspiracy theories are perfect for fanatics. As Churchill said, "They won't change their opinion and they won't change the subject."