Bohl is a better gameday coach than Mussman, that's one reason.
NDSU had to contend with fewer schools for recruits when they transitioned (UND is competing against two local already established FCS programs plus USD).
Also don't forget a couple years back NDSU went 3-8 towards the end of their transition.
UND will improve, it's all just a matter of time. Either from getting more talented recruits or by coaching changes, who knows what the catalyst might be.