Charles, it's not a super complicated problem.
The reason JFK conspiracy theories still persist is because of the EVIDENCE.
Exactly the same claim that is made for any conspiracy theory.
Those who claim Biden stole the 2020 election claim the evidence is overwhelming. Yes, it would seem unlikely that thousands of computer programmers, volunteer election workers, even Republican government officials like governors and secretary of states, would conspire together to do this, and successfully keep this secret. But, the evidence is overwhelming.
I don't group JFK CT's with other CT's like 'Q-Anon' or 'faked Moon Landings' because there's plenty of reasonable questions about the evidence in the Kennedy assassination.
I do because they all claim that the evidence for their favored conspiracy theory is overwhelming. That, yes, on the surface, a Large-Secret-Enduring Conspiracy seems unlikely. How could so many people keep such a secret? But, ‘When the evidence is overwhelming’, one must conclude that ‘The Moon Landing was fake’, or ‘The government covered up the truth about 9/11’, or ‘The government covered up the truth about the Kennedy assassination’, etc.