TBF, they tried. Back around 2010, 2011, something like that. They tried to do the PAC 16. iirc, UT and OU were interested. Somebody gummed up the works, though, can't remember who. Hooky was pretty irate the deal fell through, as I recall.
There's a lot that happened, and we've re-hashed it a bunch on this site and others.
Short version-- when it seemed like the B12 was going to implode, with Nebraska headed to the B1G, six of the B12 leftover schools started negotiating with the PAC to create a soft landing spot. Texas, Texas A&M, Colorado, Oklahoma, Oklahome State, and Texas Tech. UT athletic director DeLoss Dodds was one of the architects of that deal. All 6 of those schools had official invitations. But in the final hours, Texas A&M administration balked. They didn't really want to go to the PAC, and indeed they'd always eyed the SEC and figured if they were going to make a change, they'd rather head east. So they decided against joining the PAC. That caused a delay in the deal, and gave the B12 enough time to come up with a new deal that made the money decent enough for Texas and OU and the others to stay.
In the meantime Baylor officials began getting stirred up, and some of their politicians in the Texas state legislature started talking about trying to exert pressure on UT and A&M and Texas Tech, to get Baylor an invitation instead of Colorado. Colorado had long since been eyeing the PAC anyway, a lot of their students and alums are from California anyway, and so they went ahead and accepted the invitation before anybody could maneuver Baylor into the deal instead of the Buffs.
Ultimately the B12's new contract terms sufficed for Texas and OU, and the B12 was "saved" from destruction.
That was the 2009/2010 round of realignment.
