Not a surprise. Viewing options have increased. More teams are playing big boy football than ever, so in some areas there are multiple options - and I'd guess it's a lot cheaper/easier to get tickets for FAU, USF, and UCF than it is for FSU and Miami (for example).
Add to that the fact that with the increased viewing options, most games now have several TV timeouts, challenges, reviews, etc., so games are longer than they used to be, with a lot more downtime. For the people in the stands, that time isn't filled by much.
Then consider that with the proliferation of bowls, teams don't have to be good to get one anymore. There are 129 FBS teams, and 40 bowls, so more than 60% of teams get a berth. Bowls are no longer a reward for a good season, they're a participation trophy for being not too much worse than average.
And, even more critically, in today's digital, one-click, fast delivery world (which has always existed to today's students) they can stream games to wherever they are for less than the cost of a sports pass, skip commercials, and/or just watch the highlights (for any game) and not have to sit out in the weather for 4 hours. And the universities will let them choose that, because the schools decided that butts in the seats are irrelevant because butts on the couches pay better.