It's easy to see why this might happen. Given the sheer amount of evidence against Oswald, one has to either believe a lot of it or posit quite a lot of people faking evidence. Was there a clear paper trail linking Oswald to the Mannlicher-Carcano rifle? It must have been forged, and FBI investigators had to allow it to be introduced into evidence as though it was genuine.
Do three backyard photos link Oswald to the rifle, the pistol used to shoot Tippit, and to radical leftist periodicals? They must have been forged in a sophisticated government photo lab. And of course all the Dallas police officers who claimed to have discovered the photos were lying. Then, of course, an FBI expert told the Warren Commission that the photos were genuine. He must have been lying too. And the House Select Committee put a whole crew of top photo experts on the issue, and they said the photos were authentic. So they must have been lying too.
Repeat this analysis for other pieces of evidence, and for all the witnesses who gave testimony damaging to Oswald, and soon you have the proverbial "cast of thousands."
And this is just the conspiracy to cover up the fact that a conspiracy killed Kennedy, not the conspiracy to actually shoot the president.
Let's assume, for the sake of argument, that three people are involved in a conspiracy. They successfully kill JFK and then move on to other pursuits. Assume, further, that the probability that any single conspirator will "defect" and "blow the whistle" on his fellow conspirators is one in twenty (.05). The probability that he won't defect is therefore .95.
What is the probability then, that none of the three conspirators will defect? It's:
.95 x .95 x .95 = .857
Such a conspiracy would likely be successful. There are a bit under three chances in 20 that one or more of the members would defect.
Now let's suppose that the conspiracy has 20 plotters, each of whom must stay quiet to avoid "blowing the plot." The probability that none of them will do so is:
.9520 = 0.358
In other words, the odds are almost two to one against such a conspiracy "hanging together." If there are 30 members, the probability is:
.9530 = 0.215
But this scenario begs the question of just who is going to imprison the other 29 conspirators. The "prison" must have "guards," and of course they know of the conspiracy.
It might seem more sensible to simply kill off, rather than imprison, members of the assassination conspiracy.
But the same logical problem holds for the notion of a "cleanup squad" of people going around killing off people who might "spill the beans." The members of the cleanup squad themselves become members of the conspiracy who have to be imprisoned, killed off, or allowed to run around with very dangerous knowledge.
.9520 = 0.358
". . . exactly how big was the conspiracy to kill JFK? Are we talking about one assassin with an accomplice or are we talking about something larger? If one were to believe the current literature, we are faced with not just 'something larger' but a monster conspiracy that consists of several assassins, several accomplices, and the destruction and forgery of vital evidence. The critics have constructed a conspiracy so massive that it ultimately falls of its own weight." Fred Litwin
Of course, one can posit that the conspirators are able to suborn local law enforcement officials. If this is so, this might drive up the probability of a "success" as even sloppy murders are covered up by cops. But of course this comes at the expense of even more people who know that something conspiratorial is going on. For example, assume that bribery, threats, blackmail or such applied against five law enforcement officials will drive up to .99 the probability that any given murder can be done successfully, then twenty murders can be done with the odds of success in the total enterprise being:
.9920 = 0.818
That sounds like pretty good odds for the conspiracy. But then there are 100 law enforcement officials running around who might decide to "blow the whistle." If each of them has a .98 probability of remaining silent, the probability that all of them will remain silent is:
.98100 = 0.1326
Those aren't particularly good odds. What has happened is that plotters have purchased short-term success at the expense of a high liklihood that someone will eventually speak out.
In fact, when one looks at the details of the "mysterious deaths" they usually don't seem so mysterious. If a conspiracy killed Kennedy, it's not boneheaded enough to go around killing dozens of additional people.
The probabilities involved here are obvious by now. If there is a .95 probability that each individual conspiracy task will be properly executed, the probability that every one of a dozen conspiracy tasks will be properly executed is .9512 which equals .358. As Gary Sumner has observed in his essay "Logic and the Killing Of John Kennedy":
According to well-known Murphy’s Law, if anything can go wrong, it will. Imagine all the things that could have gone wrong in attempting a difficult, dangerous operation such as killing the President of the United States. Considering the idiotic nature of the plan, the slightest mishap, the tiniest unforeseen circumstance, could have brought the operation to ruin. But nothing went wrong. The killers achieved perfection.This sort of logic, of course, doesn't apply if one has a very simple conspiracy theory. Why not, for example, posit that someone persuaded Lee Oswald to take his gun to work and shoot Kennedy? Why not posit that Oswald, rather than being a manipulated "patsy" who might turn on his manipulators and talk, was a witting member of the conspiracy who fully understood and agreed with its purposes? Why not posit that only a very few people, or even one person, was a part of the conspiracy with Oswald?
And since then they have successfully resisted the urge to talk about it. Various authors have postulated anywhere from a couple of dozen conspirators to several hundred. At this writing the assassination took place 40 years ago, yet no conspirator has talked. Not one has gotten drunk and revealed the murder to his wife or mistress, who has then gone to the authorities or the media. Not one has made a death-bed confession. Not one has left behind a letter of explanation in his lawyer’s safe to be opened after his death.
Think about it. These mysterious men, many of whom must not even have known one another before the plot was hatched, got together, planned and carried out the crime of the ages, in public and on television, then vanished ghostlike into history. Nobody saw them and they didn’t make any mistakes. None of them ever talked. They committed the perfect crime, using the stupidest plan imaginable, and got away with it.
The reason is simple. If you do this you have to give up the notion of a Grassy Knoll shooter. You have to accept the Single Bullet Theory. You have to discard notions about faked and forged evidence. You have to let LBJ, the CIA, the FBI, anti-Castro Cubans, the Mafia and all the rest of the usual suspects off the hook. You have to quit claiming that the Warren Commission was a "coverup." Whatever the logical virtues of such an approach, it's grossly unsatisifying.