I don't think this is an overall new thing and there are a lot of places where the demand just isn't always cemented.
As a fan of big-time sports, I loved living in Phoenix. Because you had everything (at the time all four sports, plus all the big events), and there just was no real demand for anything. You could go see almost anything for not a whole lot of money or work. While I lived there I went to every Fiesta Bowl, NLCS games, the NFC championship game and the first Patriots-Giants Super Bowl. I think the latter, even in Phoenix, has become much more of a bear to get into, but at the time, it wasn't bad at all with no other costs and it was a great experience.
Anyway, I went to so many Cardinals games and I don't know if they ever had the vast majority or even anyone really caring outside of that NFC championship game -- that was the one time Phoenix really, really got behind a team while I was there. It was nuts and a lot of fun to be around. The town loves the Suns more than anyone, but that was just different in my experience.
The next year they played the Packers in the playoffs -- the freaking Packers -- and you could have gotten in for $10. I wanted to but couldn't find anyone to go with me or a babysitter. That was obviously a while ago so maybe it has tightened some there, but going to games, and definitely NFL games, was simple. The one Broncos regular season game while I lived there it was easily 70 percent Broncos fans.
Sidenote, I'll add the Broncos to both travels well and doesn't let the other team in. Almost always, but definitely when good. The one exception to visiting fans might be the Steelers, but even then Denver does a pretty good job. Now the other Denver sports, not so much, especially with Chicago and Boston teams, and the Lakers.