BYU will never be in the conference. XII tv partners want to have the freedom to play post season tournaments on Sundays, and BYU will never play on Sundays. If not for that, they'd already be in.
The problem with UCinn in football is that like many urban university's with major league sports, they don't support the local college teams, except for event kind of games. Miami is the same way, though they hide it well.
Miami traditionally fills up the place to play Florida State. But the stadium isn't half full for lesser opponents. Cincinnati is a pretty small city compared to many that have both Major League Baseball and NFL football.
It would surprise most people that Cincinnati's population is almost 100,000 smaller than Tulsa. Tulsa is also bigger than Cleveland, which also has MLB and NFL franchises. Heck, Arlngton Tx is a suburb in the DFW area, and it is bigger than Cincinnati and only about 30,000 smaller than Cleveland. Look at SMU essentially in Dallas, which seldom has 10,000 in the stands for home games. Excluding students, less than half that. Allen High School outdraws SMU.
There are exceptions. UDub draws pretty well, even though there are MLB and NFL teams in Seattle. But it's a state school. Same with ASU in the Phoenix area. But it is different when the alumni base is a lot larger.