I think trying to pinpoint the reason for a sport's long failure is pretty difficult. If I had to start, it'd be Fred Glass isn't a particularly good AD. McRobbie doesn't give a damn about sports, and he shouldn't. He's the president of an academic institution, and sports are intended to be secondary. A recent ranking of athletic department spending had IU at No. 9 in the B1G, between Illinois and Minnesota (
link). If you're trying to solve success, spending may be the best guess.
I do think, and it's been mentioned on here before, that IU is a basketball school, and however that materializes (coaches' salaries, facilities, fan support, recruiting advantage) does seem to matter. As an Ohioan that graduated from IU, I was blown away by the support for the basketball team. The only thing I could compare it to was Ohio State football. And I think IU football suffers for that. I was also mildly surprised to see how little interest there was in football. Going to a football game was a lark ("Never lost a tailgate!"). Nobody expected much, nor really cared. It was a reason to socialize. A win was a pleasant surprise. I do think some sustained success would gin up the energy.
But the inability to string together successful football seasons means you have fewer diehards, few folks that keep coming back because of that legacy, that 10-year run where they dominated the conference. It probably goes without saying, but every successful season produces lifelong fans. Multiple successful seasons compound that interest. Bigger success does the same. Since they've never really distinguished themselves, they have very little vested interest in the program; fewer folks willing to buy season tickets or sit through an expected loss. They don't have anything to buoy their spirit to. They don't recall how exciting it was when the team was great while they were in school. It's an afterthought for the vast majority of IU fans.
And I suspect it'll remain that way. It's harder for the football program to get out of the shadow of basketball. Everybody pegs the IU fandom to basketball. Football success is a nice surprise. And because of that and their inability to create sustained energy around the program, it's a steep climb.