Firs, humans have an extremely poor innate understanding of how chance and probability work ( a very common one- if you are flipping a coin, the chance of it turning up heads does NOT increase if you have a long series of it turning up tails: the chances will continue to be, for each flip, 50/50... but most people will feel it just should be otherwise). Given that, humans tend to see agency in random events- think something was caused by somebody. And, the greater the event, the greater the willingness to believe in a greater agency. The preponderance of the evidence in the JFK case is: Oswald. He owned the gun that was used, which had his fingerprints, and was found at the Book Depository, where he worked. And he tried to flee, and killed a police officer who just happened to walk in front of him while doing that. Many people have pointed out that, as crimes go, it is very much an open-and-shut case. But it has been very hard for some people to accept that Oswald just had a modestly lucky day, JFK had a very unlucky one. It seems there should be far, far more involved. There have been other , similar cases: for example, the conspiracy theories around the Twin Towers collapsing from something other than the airplanes striking. There should be something more to it, we feel, than bad luck.

There have been a number of pretty good books weighing the evidence and weighing the conspiracy theories. ( My personal favorite is Vincent Bugliosi's Reclaiming History, despite the author's hectoring writing style). There is a common problem with the JFK conspiracy theories in that while they demand incredible uniformity and consistency of the witnesses and evidence that was gathered to prosecute Oswald, they ignore the immense, obvious problems with the evidence for their own theories. The famous Mafia/Jack Ruby theory, for example, ignores the fact that Ruby happened on Oswald's transferal and shot him- he had not staked out the Dallas Police building to wait for the chance- so there's no way he could have planned it, and thus could not have been sent by the Mafia. Moreover, the transfer happened an hour later than had been announced. At the scheduled time Ruby was patiently in line at the post office, getting a money order.

This sort of problem is common for advocates of unconventional theories, generally. Erik van Daniken spouted hundreds, perhaps thousands of them, all asking us to question whether, say, Bronze-age farmers could have built the pyramids, considering how primitive they were...but then leaving unanswered the immense problems of where the space aliens went, and what evidence do we have of them, if they built it. This technique ( in van Daniken's case, you have to call it a technique) is usually pretty effective: demand somebody explain how the pyramids were built, down to the last bag of dirt; and when they can't , triumphantly conclude it had to be space aliens. Much easier than trying to explain how and when the space aliens came, what they wore, why they bothered to push stones around, when it was they left, what traces of them exist.