The whole thing is like two siblings who don't get along. This thread has turned into bickering. Leave out hockey, wrestling, and swimming and they're two programs that are very similar in terms of support, sucess and athletes. A former Sioux FB player told me that the only difference between a Bison and Sioux FB player is the color of their jersey.
On that note, I think the Bison won't play the Sioux for the same reasons the Sioux wouldn't play the Bison a few years ago, playoff consideration and spite. Bison need to be voted in for playoffs and strength of schedule is important. Doesn't mean there can't be games in other sports though. I can understand NDSU not wanting to get burned again, but I think they should have taken the high road, sucked it up, and put a few games on some schedules.