My wife and I walked by the OSU stadium yesterday, they were preparing to play UC Berkley or "Cal" in a Pac 10 football game. My wife asked me why SIU wasn't in one of the big time football conferences. Had to think about this a bit, but what I think is this has to do with SIU's jump from being a "normal" school to being a full university after WW2. The big football conferences were formed the late 1800's and early 1900's, and SIU wasn't a university then.
OSU had the same growth after WW2, almost exactly the same story as SIUC. They have the world's biggest HP plant there and are only 70 miles from Portland and 30 miles from their state capital, so they are way less rural and remote.
Strangely enough, the big time football schools really don't want to have all the little universities join them in the big money. Also, there is a lack of tradition, TV coverage, and quality opponents every week (like Big 10, Big 12 or Pac 12). It is a tough climb into the football big time.
In Central Oregon the nearest NFL team in 300+ miles in Seattle and Seattle is a fairly new NFL city. Here in Southern Illinois we have always had St. Louis, and our basketball and pro baseball (Cardinals and Cubs) culture. Tradition is important to sport fans and building one is difficult.
I wonder how many years it will take to draw 15,000 fans to a SIUC football game, even though 20,000 students have free tickets?