I'll tell you why:
1. Evidence is strong that Communist Kennedy-hater Lee Harvey Oswald started work at the Texas School Book Depository long before it was known that Kennedy would be driving past. On the fateful day, Kennedy did not take the usual route for a Preisdent visiting Dallas, so Oswald could not have possibly taken that job hoping Kennedy would drive past.
2. Evidence is strong that Oswald shot from his workplace, at the Presdent, at a distance and with a weapon that one could reasonably expect to kill the President. And then the President died.
3. After the President's death, vast numbers of liberals just could not accept that a Communist, whether acting alone or with a moderate amount of Cuban government help, had killed the President. So they tried like defense attorneys to poke holes in the case against Oswald. Most of these conspiracy theorist just cannot accept that the Communist threat was real, so that had to look for holes in the obvious explanation for Kennedy's death.
As a conservative, it is easy for me to believe that this assasination was a communist hit. Then I read the story of Oswald's life, and it all comes together. The liberals don't want to believe the truth about what communism did at Daly Plaza. And a few conservatives have been taken in by an anti-government gloss being put on an essentially liberal attempt to blur the motive for Kennedy's death. Kennedy was killed because he asked us to "bear any burden" in fighting the ideology to which Mr. Osward and millions of other evil ones were committed (and, in North Korea, are committed. This is what should be remembered first when one remembers the Kennedy assasination.