Bigger games, bigger stadiums, bigger stakes, bigger players.
Going to the SEC was huge for Texas A&M. It may not be as big for Texas and OU, but if they want to compete at the top level, they are not going to do it in the Big 12.
The consolidation of football has been happening since the 80s, with the better conferences getting better teams to join. Big East was kicked off the BCS ship, now it looks like the Big 12 is the next up - even though it was the 3rd best conference in revenue.