For one it keeps you from looking like fools on a national stage.
In addition to avoiding looking completely inept you can squeeze 10MM each out of the deal before the GOR runs out.
It's been reported by virtually everyone that no one could agree on the teams involved, so what's the point in fighting if not everyone else could agree either? Secondly, they already look like fools. And if they expanded only to leave in 6 years, they'd still look like fools. It's for that reason it's been reported they didn't want to "squeeze" more money out of the networks because everyone would realize they're just looking for a short term payday. At this point there's really nothing to gain. The networks were fighting back, the schools were fighting over who to add, what's really the point for OU and UT to challenge this if they truly know the Big 12 is on its death bed?