Simple, he got shot. America tends to make martyrs out of everyday people. I truly believe that had he not been shot, he and the rest of the "Kennedy Dynasty" would be in the dust bin of history. Had Teddy not been a Kennedy, and had his brothers (plural) not been shot, he would probably be in jail.
Then maybe by that logic James Garfield should be our greatest president, as he got shot before he had a chance to screw up anything at all!