u/eternalkerri led a good discussion of this here some years back. For most historians, it's not really muddy. It was Oswald. In a place with a good view, with a rifle that worked, on a target that was in range.

As for why it remains muddy for many people, I would suggest much of it is due to the fact that humans have a very poor intuitive understanding of chance. For a significant human event, we want there to be an equal significant cause, hopefully a human cause. Simple explanations for the success of the surprise attack on Pearl Harbor were also long discounted in favor of conspiracies and intrigue, and the disappearance of Amelia Earhardt over the Pacific is now getting attention for possible Japanese involvement.. It feels wrong somehow, that a lone loser like Oswald should have had a moderately lucky day, or that the bright and beautiful Earhardt should have simply gotten lost and run out of fuel. Like having a fairy tale that says, "One day the prince got up out of bed, strapped on his best sword, walked out the front of his castle and was hit and overrun by a passing cart and horse, and died. The end".

Oswald did, the evidence is very strong. The gun, the shots, the training, the motive, the location of the shooting, the injuries, etc.

There are a couple of reasons why I think people have a hard time accepting that simple truth. Starting with the common human weakness in having exceptionally poor intuition in matters of physics, and in logically and rationally reconciling that poor intuition with reality. Look at the simple fact that it took hundreds of thousands of years for modern humans to achieve an understanding even of ordinary motion (something humans have constantly interacted with on a daily basis for all that time) in a way that was reflected reality to a reasonable degree. People fixate on the "magic bullet theory" don't actually investigate the physical reality of the situation and then veer off into alternate theories even more untenable and unsupported by evidence (grassy knoll, etc.) The reality is that when you position Kennedy and Connally where they were in the limo at the time of the shooting, positions supported by multiple items of photographic evidence, then the bullet trajectories and the wounds inflicted by them are easily explained. And, in fact, the shots are reproducible. They may be surprising trajectories for people unfamiliar with terminal ballistics or the laws of physics, but they are nevertheless real, physical trajectories.

That, however, is really just the crack in the door that lets in the conspiracy theories. The reasons why there is such pressure for alternative explanations are different. For one you have the context of the situation. Dallas at that time had earned a reputation as a "city of hate" against Kennedy, the Democratic party, and progressive causes in general. When JFK visited the city and then was struck down the immediate reaction by many who liked the president was to blame the city, blame the establishment, blame the environment of hate, etc. When Oswald was revealed to be just some random guy who sorta kinda hated America and sorta kinda had Communist leanings but not terribly seriously, it threw a wrench into that whole reaction. On top of that there is the persistent human misconception that major events and major losses only happen for significant reasons.

Layer onto that the other complexities of the situation. Oswald never received a trial, evidence was never presented in court to convict him of the murder, because he was shot by a civilian while in police custody. Not only was that a very bizarre situation, it also led to a very unsatisfying outcome: a lack of an official legal finding of guilt. The Warren Commission would produce a report detailing the evidence and producing a finding that Oswald was the killer, but in many ways this was unsatisfactory to the public, and perhaps even by then conspiracy theories had begun sprouting up in people's minds.