The current deal with the Big12 includes a clause that does not allow for renegotiation of the contract if the Big12 expands, in exchange, the Big12 is guaranteed that they can add up to 4 teams and the payout to the Big12 would increase for each team added by an equal share for the current teams. Ex: (Easy math only, I don't have the payout schedule handy) Assume the payout this year is $200MM, or $20MM/team.. The Big 12 expands by tow they would not receive $240MM, still $20MM/team. If they expand by four, the payout would be $280MM, again, $20MM./team.
As this is contractual, the networks cannot "block" the expansion outright, they must have cause, for the first four teams. It has been brandied about that if the Big12 expands by six, they would be able to re-negotiate their current deal. However, ESPN is balking at the expansion now (they stand to lose teams under much lesser agreements), it stands to reason that they would refuse to pay even more for the loss of up to six teams.
Fox wants the expansion as they need game inventory. ESPN owns most of the properties that the Big12 is truly interested in so they would be giving up properties (AAC, CUSA) to Fox and would be paying significantly more for the same property under the Big12 deal.
UT and OU want to remain relevant so they must determine whether the Big 12 is sustainable long term or to jump (probably at the end of the current GOR - 9 seasons).