To enter the game is free. The charge is for athletic fees, a sunken cost. If they go to a game, they don't pay any more. If they don't go to a game, they don't pay any less. There is no tradeoff. You pay taxes to upkeep things like parks whether you go or not. While the tax is collected be it on state, county, park district level you don't pay an individual tax for every one of them.
And perhaps there is a perception of "free" by the students. They don't value the game since again, they already paid for it fees and/or aren't payin any more. They have no need to attend to justify the $. That is true of many events, goods in general. I have gotten plenty of free tickets for small crap and never went. I, and many others I am sure, have also picked up a lot of crap at booths such as festivals and never used. But if you paid for it, you are more likely to use. Food/drink is also a good example.
I am not saying it would work. But if you are lookiing to increase attendence AND are willing to gamble on losing guaranteed $, at least short term, it's worth a try. Lower fees by X amount, then charge the same to get in perhaps. Have more gimmicks, albeit our last one with the 4k students a game or whatever to get year end prize didn't go so well.
If they truly cared about boosting student attendance, do something. I mean NIU is a good football program now and somehow is losing steam. Everyone commends the party atmosphere the students present right outside the stadium. Does it really help? I don't know. But many then condemn them for not going and/or not staying. I'm not in marketing/sales but it seems I and many others on here could not do any worse to get students to the game.
I guess except those who point to the budget as an excuse for everything that goes wrong and NIU and the State in general.