Back when there were 10 it was the Big 2 Little 8. They didn’t like that so they messed with the numbers.
That's before recruiting was completely nationalized. Regional and state ties were important and even the elite programs relied heavily on in-state recruiting. The two big population states dominated the small population states. Illinois should have been able to hang with Michigan and Ohio State except that Chicago had no particular attachment to the rest of the state and was the happy hunting ground for the whole league.
Television dollars and the erosion of academic standards have changed college football beyond recognition, and not for the better.