Because adding Cincy is going to stop OU or Texas from defecting. Did adding Cincy to the Big Least stop Pitt, Syracuse, UL or WVU from leaving?
OU and Texas aren't trying to leave the BIG 12, so Cincy isn't being brought in to stop them or anyone else from leaving. Schools are being considered for expansion because the BIG 12 conference needs to eliminate disadvantages and improve things like finances so that the BIG 12 will remain at the top competitively and financially so that no one will consider leaving in the future. There won't be any reason to consider it.
If the conference doesn't change anything between now and 2025 while everyone around them continues to be proactive, then there will be problems.