Here's a better question: If Oswald was the lone assassin, then why are the records sealed until 2040? Answer this question and the riddle is solved.
I probably won't live long enough to see the JFK assassination records unsealed, but then again, something tells me when 2040 comes around, assuming the USA is still here despite our headlong pursuit towards Socialism, I believe the can will just be kicked down the road a bit farther. If there wasn't a conspiracy to kill the President, be it involving LBJ, Cuban, the mob or whoever, then unseal the records now and end the speculation once and for all!
There is nothing in those records that is going to read: The CIA, or LBJ, or the Martians killed JFK. If a conspiracy was behind the assassination, which I seriously doubt, they would have been eviscerated in 63/64.