I hate to say a big reason is because the SEC has dominated the sport the last 20+ years. Non-SEC fans have SEC fatigue. Heck, most of us do too.
I'd say that's pretty low on the list, certainly behind the fact that every game is available in HD, big-screen TVs are much more affordable than they used to be, and maybe the biggest reason: ticket prices, which were high 20 years ago, before the Saban era ignited the arms race that has led to ballooning budgets in every P5 program.
I went to 2 games this past season. There was a time when I wouldn't have missed a home game. It's just too much money. If my wife and kids go with me, it costs almost $500 for a single game by the time you factor in parking, concessions, etc. I love the pageantry of a college football gameday. But more and more I'd rather be at home with 3 different big screens playing 3 different games, a hunk of meat in the smoker, and a few friends over. Way cheaper and, to be honest, probably more enjoyable.
And, while it does make sense to reduce the capacity of Neyland as Vinnie said, the end result of that is going to be even higher ticket prices for the marquee games that people actually want to attend.