1. They get more money. A lot more money. 9 figures more money over the next 8 seasons. This is contractually guaranteed and irrefutably true.

2. They improve their chances of a CFP berth. This is based on research done by consultants the Big12 hired because the perception in the media, with recruits and with their own institutions is that being at 10 has hurt them. They've already let the cat out of the bag that expansion significantly helps their chances. They can't walk that back. If they don't expand, perception is only going to suffer and IMO they can't afford the fallout.

I have yet to see anyone provide any reasons why #1 and #2 no longer hold or how there have been any viable alternatives that give them more money or more access to the CFP without expanding.