Some people simply need there to be a conspiracy. Maybe it makes them feel better.
In some cases, that may be true. The world is complicated and in some ways, conspiracy theories help us make sense of complicated events that are difficult to explain.
However, JFK assassination conspiracy theories persist due to the evidence and the lack of transparency from the US government. I mean, if Oswald was just a "nobody" and the case is so "straightforward", why is there so much secrecy around the JFK files almost 60 years later? I have yet to hear a satisfactory answer to that question.