Injuries. And a lack of speed. If you're big and you want to win a major, you have to do it young. Safin (6'4) won his first slam at 20, but did win once again at 25. Delpo won USO at 21. Phillipoussis had a late run on grass when he was 27, but all of his big results came before he hit 24. Otherwise injuries hobble all of the big serving, big hitting big men as they age. Another way to look at this is that big men get slower before they can dominate. The real question is, can their games improve faster than they slow down? Safin and Ljubicic were statues for most of their careers. Delpo is heading in that direction. Raonic will soon. Even Murray, who is famed for his speed, is really only fast for a man his size.