I always wonder what the Central Illinois/Chicagoland/Other ratios are for our basketball crowds vs our football crowds.
My anecdotal sense is that the hardcore Central Illinois fanbase who have relatively quick and easy transportation to the games fills SFC but doesn't come even close to filling MS and thus the marginal attendance needs to come from further afield. (Braggin' Rights versus football games in the St Louis dome having a similar effect, we have a dedicated fanbase in that area, but it's only so big)
My other pet theory is that all the hotels in CU having two-night minimums on football weekends is an underrated contributor to the inability to attract those out-of-towners. It is certainly a negative impact for me personally.
Also even just the price. If we are going to come for a football game, the wife and I prefer two nights anyway … but when the Hyatt (only cool hotel Downtown to my knowledge) is charging $700 per night, no thanks. Thankfully this past year for PSU, we found a great Airbnb option less than a mile west of the stadium. However, it is my perception that the Airbnb scene is still VERY underdeveloped in Champaign, considering it’s a major college town with like 250k people in the metro.
I also find your first question fascinating myself. My dad once said (with no evidence, just his gut feeling) that you should assume Illini fans in a given location are taking advantage of seeing the team while they're close. We never missed an Illini basketball game in Iowa City growing up, and I always found it interesting thinking that a lot of the fans in attendance were Illini fans from the Quad Cities who had a much easier time getting to Iowa City than to Champaign. Similarly, I have always imagined the crowds next to me in Evanston as 90%+ Chicago Area fans that are catching a "local" game and the majority of the fans around us at Braggin' Rights being from the Metro East and Southern Illinois.
With this preconceived bias in mind, my wife and I always have a little bit of fun trying to gauge how different the Illini contingents seem in Iowa City, Evanston and St. Louis, as we have seen them play at each place multiple times now. You can definitely notice a bit of a difference, haha. As for your question, my TOTAL guess is that the vast majority of our regular basketball fan base at home games is from C-U or the surrounding counties. I would imagine that our football "floor" attendance (which seems to have stabilized up to about 45k now) is similar, but probably more from Bloomington, Springfield, Peoria, etc. However, I would guess that when we are pulling in bigger crowds that are pushing 55k, it is at least as much due to people from Chicagoland, St. Louis, Rockford, etc. deciding that a trip is worth it than it is from folks in Central Illinois who previously had zero interest attending a game and now because we went from bad to decent, they're in. I would kill to get my hands on some DIA data of tickets by county of the buyer, though, haha.