Other thing is that SDSU football is kinda "stuck". Not saying they're in a bad position by any means. But they've pretty much upgraded all the facilities they possible could .... new stadium, new practice facilities, new indoor field.
They were never a historical winner in DII, but they made the investment back in the early 2000's to jump up to DI-AA with NDSU. But for the most part, they just can't seem to get past NDSU. They do everything they can, they have good players (guys getting drafted even). They recruit well in the upper midwest, as well as Nebraska and Iowa. They just can't get over the hump.
Brookings is a small town, cold and flat, in the middle of nowhere ... pretty much a junior version of Ames. That turns some people off, of course.
UNI was tough for a while, but now NDSU is almost the automatic conference and then national champion every year. And South Dakota is even starting to catch up. UNI is still tough. UND coming in will be tough. IL State is tough.
Is what it is. Just an extremely tough conference at that level, and the champion (NDSU) usually goes on with little competition from the rest of the nation in the playoffs.