He dove because there was a Cardinal at his feet when he got the ball. It was do or die.
One thing I wanted to point out is that after the game was over the announcing crew were asking why they didnt pass it and give the QB some options.
This strikes me as a typical MMQB move second guessing the coach's play calling. Had they thrown the ball and not connected they would have been crucified with the announcing crew saying they should have ran the ball in from the 2 yard line.
Personally I think that the best call would have been to play action waggle out to a TE like Vernon Davis or whomever.
The Cardinals have a decent defense with Darnell Dockett in the middle and some big boy linebackers.