U said it yourself. They already have those school. Why add another?
You add them if you think it expands the pie. Since they got an exception to be allowed to play a championship game, getting back to twelve teams may not help at all. They might actually have the best situation right now, able to do a 9 game conference round robin and a conference championship game.
Or maybe that's still inferior to a 12 game conference because it cuts down on the number of buy games, which cuts down on home games and ticket sales for their conference? I would think since most schools end up doing an away game non conference anyway that doesn't really hurt them. But they may view a 12 team conference as more stable, and they would still add a tv market, albeit a small one.