A lot of it is what plagues football all across Yankeedom. School segregation. They can't do that in the South and the government isn't too keen on going too overboard on "economic segregation" so a lot of the South has very integrated schools with busing and open enrollment being the norm. This means that the kid with the single mom is gonna have plenty of friends with a dad around and those guys get all the kids playing football and getting into Scouts and stuff like that. That simply doesn't exist in places like Chicago, Milwaukee, New York, Detroit, etc. Des Moines is on a much smaller scale, but the same issue arises. Once the SCOTUS green-lighted "de facto" school segregation in the 1970's, the northern urban cores were pretty much toast in terms of school quality and football is just a readily discernible manifestation of that.
In the South, even if you grow up poor, the community is gonna get you playing football. Maybe you'll have to do some fund raisers in front of Wal-Mart, but you'll get out there. That community is not present in the North and if kids are poor or a different color, no big deal as long as they live across town and don't "dilute" the "good" schools like Valley, Ankeny or whatever.