Because he's fat. Duh.
But you look at all the greats modern hitters, they were able to steal, but then decrease that and keep their hitting up. Bonds, Gwynn, Afraud, etc.
Might also be a durability issue. Going all out, sliding into second. Then having to go back because of a foul, over a course of a year, and even a career, is very wearing on the body.