I have thought about this for a long time and I've observed the following:
1) Competition from the internet, cable/satellite TV, cheap airline travel, etc. There is just more accessible things to do now.
2) Tulsa isn't growing as a city. Let's face it, people leave Tulsa, they don't move there anymore.
3) TU students move away for jobs. How many of us on this board are in other cities/states?
4) Low enrollment equates to a low donor and fan pool to draw from. The Administration has never been willing to do what it takes to get enrollment above 10,000. There were 7,500 when I got there in 1987 and there is under 5,000 now. "Elite" schools usually do not have an acceptance rate above 20%, TU is at 41%. I would like to think this is the University doing an efficient job of targeting potential students. Whatever the reason, it means there are only about 2,100 applicants a year which is 1/3 the number of applications to Rice (which has an undergrad enrollment just 500 more than Tulsa). The "Tulsa" brand just isn't that big.
Wait a second! Rice sucks at everything (except baseball)...
Yes they do! And so do many other schools that are on par with Tulsa (SMU, Wake Forest, Boston University, Tulane, Fordham, Marquette, etc.). Which means (in my opinion) that Tulsa has enjoyed a higher level of sports success than it should have in Football. The fact there are 17,000 people at a game on a holiday weekend where both OU and OSU are playing home games within driving distance is amazing to me. Many of the schools I mention above don't even have football teams. The ones that do we can probably beat.
So really, we aren't doing so bad.