TV numbers and look down the road a little.
Both the Pac-10 and the Big 12 are going to get a lot more money from their new deals.
If Texas thought it could make more from the Pac-10's future, still-to-be-negotiated deal than from the Big 12's future, still-to-be-negotiated deal, then logic dictates that they would consider it.
Yes, Texas currently makes more than USC from TV revenue. I know that, having posted my research on that very topic previously (see, for example this post).
But the current TV revenue is irrelevant to the future, and only represents a portion of Texas' total football revenue (albeit a significant portion).