I still don't see why the other for P5 conferences would let the fifth get a special arrangement... here is a good point posted in a comment section:
"The Big12 will have a huge advantage for the playoffs if the above proposal goes through. The conferences currently in a 2 division format (the ACC, Big10, SEC, etc..) cannot insure their top 2 ranked team play in their CCG. For example, FSU and Clemson are in the same division in the ACC and OSU and MSU are in the same division in the Big10 so they by design they cannot play in the CCG even if they are the 2 highest ranked team in the conference.
If it comes down to "strength of schedule" or "quality wins" for that last spot in the playoff, the Big12 has an advantage since it always can match its 2 best teams.
To give this kind of advantage to the Big12 the other conferences must really not want the possibility of expansion."
The last point the poster made is imho right on point --- and I just can't see the other conferences being that dead set against expansion.