Money, the same reason he's going to make the Redskins fire him vs quitting. On top of that he's won Super bowls, has big name recognition, and could get a percentage of the Texas tv network thrown into his contract, like they were proposing with Saban.
I'm not sure why Gruden wouldn't jump at this massive pay day too considering the Big 12 is really the little four... OK, OK State, Texas and Baylor and that Texas is considered the hotbed of high school recruiting.
This is a very attractive job with tons of perks!