NIU has also traditionally been one of the lowest ranked MAC teams in recruiting ranking. It would seem that they may get a lot of under-the radar guys from Illinois and Wisconsin? The rest of the MAC seems to beat itself up scouring Indiana-Ohio-Pennsylvania for under-the-radar types, while NIU may get Illinois-Wisconsin-Iowa all to itself as there are no other non-BCS FBS schools in that area.
Sure enough, I looked at the recruiting, and they get most of their players from Illinois and Wisconsin, and usually the only schools they are competing with are Bowling Green, WMU, and Ball State, plus FCS schools like North Dakota State, S.Illinois, E. Illinois, and W. Illinois. When you are able to cherry pick the best of the talent that slips beneath the BCS schools, it really helps. Ohio has been able to do that with Nebraska, and it has helped a lot. NIU apparently is able to do that with Illinois and Wisconsin.
This is why I've always found the MAC to be so intriguing. Instead of 1-4 teams searching for talent in Ohio, Michigan and Pennsylvania, the MAC brings what, 10? After OSU, Michigan, MSU, PSU, Pitt, WVU and Cincy get their guys, there are still enough scraps left over for the MAC to breed good competition.