Could just be about teaching a lesson, like dropping Baylor due to non-conference scheduling. "Bring us an actual champion next time and we'll consider it," just like "if you had scheduled ANY P5 school you'd have been a few slots higher all year."
The bigger issue than actual size of the market is the amount of the market they deliver, and I don't think that is very large with Cincinnati. The size of their stadium, for example, isn't as much of a concern as the reason why their stadium is so small, which is because they don't have a particularly large and devoted fanbase. Having a lot of students doesn't necessarily equate to having a lot of fans of your football team.