As always, it's about the money.
But if I'm DeLoss and Joe, I fight like hell to keep the Big 12, it's a sure fire ticket to keep both of their schools at or near the top of the heap year end and year out.
No other league offers that. There's a ton more "sleeping giants" in the Pac-12 than there is in the Big 12, and in the SEC, forget about it, they'll win that league once every so often, that's about it.