I mean, it’s kind of easy to make a conspiracy theory out of it when the assassin is killed and his killer dies of lung cancer 2-3 years later.

The warren commission also had its own issues; including lack of motive - https://www.history.com/news/9-things-you-may-not-know-about...

Then the government agencies in the 70s are caught doing a lot of illegal provable (often admitted) conspiracies. These including things such as spying, giving citizen deadly diseases (Tuskegee experiments), blackmailing, bribing and threatening members of congress, etc (see church committee)

It’s easy to see why these conspiracy theories persist.