Probably because he realizes that Obama is the smartest, most talented guy in the race. I happen to have a friend who's a Harvard Law professor, and he's one of the smartest, best-educated people I've ever met. He says that Obama is one of the smartest, best-educated people that he's ever met. Like my friend, Buffett knows talent when he sees it.
Any thinking voter will realize that no other issue matters right now - The very fate of our economy hangs in the balance. I don't envy the person who inherits an $11 trillion debt on January 20th, but I'm willing to cast my lot with the Harvard Law grad, who's backed by the world's most prescient financier.