Because their stadium seats 19,000. FBS teams must have a 30,000 seat stadium. I'm guessing they do not want to make that large of a financial commitment. We average over 40,000, get a much larger payment from the conference and are financially struggling.
Stadium size isn't an issue. As long as your stadium can accommodate enough to help you meet the 15,000 average home attendance requirement (which isn't enforced anyway), your stadium can have as many or as few seats as you want. Recent FBS move-up ODU wants to build a brand new stadium, with only 22,000 seats. FBS conferences may tell NDSU that they want to see plans for a larger stadium before extending an invitation, however, but it's not an NCAA requirement.
The problem NDSU has is that no G5 conference wants them. All G5 conferences have 12-14 football members and don't need any more schools to split declining revenue pools with. Also, NDSU is far, really far, from any G5 conference footprint. The nearest G5 school is NIU, 600 miles away. The MAC's entire conference footprint isn't that expansive. NDSU is over 800 miles from the nearest MWC school, Wyoming. Travel travel costs matter a lot, especially in light of the shrinking revenue problem. NDSU is not in a major media market, not in a hot recruiting area. From the perspective of G5 conferences, the value isn't there.
And a lot of FCS athletic directors, presidents, and fans prefer the FCS. They hate the idea of moving to FBS to increases schollies for football and women's sports, COA, all of the added costs with not much more revenue, all for the payoff of playing in some obscure, sad bowl. Many would prefer to keep their regional rivalries and to play in a real championship playoff. So it's possible NDSU leadership and/or fans aren't interested in FBS anyway. They probably get more notoriety as the "lil ol FCS team that could" than they would as "just a good MAC team."