The AAU is about graduate research. While UND has many graduate programs, all good, it is not a graduate research focused institution. Rather its main deal is a very good basic college education.ND is head scratchingly not an AAU member. It really is an official criteria to be invited, but perhaps there might be an exception made.
ND's value would give the ACC the ability to re-negotiate a better TV deal as a full member.
Same can be said about many private schools. Many see their mission at aimed at just being the best undergrad college they can be. Two different things.