Speed and injuries. Large number of hockey players have their careers cut short by injuries, or the injuries cut down on their effectiveness. Lindros went from a beast to a floater. Orr was a shell of his former self the last few years. Kariya, Marc Savard, Pavel Bure, LaFontaine, Cam Neely, etc, etc.
It's the speed of the game the leads to the injuries in the first place. Concussions and joint damage mostly.