The Big Ten doesn't accept non AAU members. Therefore to claim academics has nothing to do with college conference affiliation is more than a bit naive. The ACC has pretended that academics were important and the new SEC commissioner also recently referenced academic credentials being important to future expansion desires.
Fans don't want there to be a connection, but for the most part (see Louisville-an act of desperation on the ACCs part) there is.
I've followed the ins and outs of expansion for quite awhile and all the information I pass along is publicly available. Many leaders in the conference have spoken out about the desires for expansion directly, or informed media spokesmen about it.
Southern Miss is not a good candidate because of many things. They have relatively 0 national brand. They are in a very small market which doesn't appeal to tv executives. They aren't particularly good in any sport-although they at times have had success at the lower levels of college football. Academically the school doesn't rate well. They don't have a particularly large fan following. Nearby air transportation may be an issue. There are lots of criteria the BIG 12 is interested in for expansion and I can't think of many if any points Southern Miss hits.
Nebraska is not an AAU school. Granted, it was when it began talks with the Big Ten in leaving the Big 12. By the time the move was made, the AAU had already signaled it would remove Nebraska from the AAU for failure to maintain standards. The Big Ten knew this before they invited Nebraska to join the conference.
Notre Dame is not an AAU and the Big Ten has courted them publicly at least 3 times in the last 30 years.
But the final word on academics for the Big Ten is from Jim Delany's quote in 2010:
"It's very important," Delany said. "AAU membership is an important part of who we are. It was an important part of who we are [when the Big Ten added] Penn State, and it's an important aspect of what makes an institution a research institution, an undergraduate school, a school that serves the public at a high level."
Asked if AAU membership was mandatory for expansion candidates, Delany said, "We're not there. I'm not going to qualify or disqualify, but it's a very important factor."
Conferences do take the academic pedigree into account when they are trying to sell a prospect to all of the voters or to exclude a school that otherwise has everything they want but also has some flaw that they deem unforgivable. WVU certainly knows the latter aspect in its dealings with the ACC.
To think academics wiled some sort of veto power in an athletic association is unsupportable. At best such things are tools to bribe or exclude a prospect.
Again the issues you have Southern Miss comes down to "they are not sexy" to you. That is not good enough. There is no practical reason they are not good enough, you just don't like them. In my dislike for BYU is stated why - they don't play in Sunday. That matters to a conference, especially one spread across 2/3s of the nation when travel must take place outside of class schedules if possible.
Southern Miss has no such negatives. Airport? Morgantown has no airport of viable worth. teams fly into Pittsburgh and bus down. The distance between Morgantown and Pitt is 82 miles; Hattiesburg to Jackson is 90 miles and Hattiesburg to Mobile is 96 miles. WVU seems to do just fine with no airport in the Big 12.