Tickets to college games are a shitload less expensive than NFL games, and they've got tons of students going to the games. You can see the trend with what's happening in new NFL stadiums between the increase in number of luxury boxes, PSL's, and just overall price hikes. They've probably done the math and 70-ish thousand is around the sweet spot to keep the stadium near capacity, keep prices high, and lower the risk of a blackout if the team sucks and people stop paying the ridiculous prices. A 90,000 seat stadium is great when the team is good, but once that team puts up a few back to back losing seasons it's probably a real burden.