I don't think "fit" matters as much as it used to. Years ago, when people basically stayed where they grew up in and didn't go far from their childhood homes when they moved, it made sense to have strictly regional conferences throughout. Today, people don't go out and say "Boy, I would love to beat the pants out of School Y because they are only a hundred miles from me and I can brag about it to their fans." Sure, you would see that behavior in the SEC fans and Duke/Carolina fans. However, casual fans (which is what conferences are aiming for more and more these days) aren't as concerned about beating neighbor schools but seeing "name" programs on the TV. That is what Notre Dame has shown us. It doesn't matter if you live in New York or Tuscaloosa. You watch Notre Dame to either cheer them on or hope that they lose.
Cuse and Duke having the ratings that they did meant that those two were drawing people in who aren't already alums of those two schools. That and ESPN being a huge giant promotion machine for the ACC has helped the conference stay in the spotlight.
Now as to the Iowa State example, my explanation is that, knowing the recent behavior of conferences, fans, TV, etc, if someday the P5 becomes a P4 that the poaching conferences would prefer taking in a school already in a P5 conference instead of a G5 school, unfair or not. TV executives often have too much of a say, which explains why UConn is still out of the ACC. Of course, I would prefer UConn to Iowa State, all things being equal. However, we're dealing with administrators and people in the academic elite circles. Fans don't get a vote on who to admit to a conference.