It's a combination of a lot of things. GenY and GenZ being attached to phones instead of humans is only part of it.
The sport could have salvaged at least some of this a decade ago, but they were all too happy to sell out their sport by increasing ticket prices and parking prices and required donation levels to the point that the average family of four was completely priced out. Even if you WANTED to drag your young kids out of the houses, off their phones, to experience a ballgame, it simply wasn't affordable for the majority of people. And if parents aren't taking kids to games, then new fans are not being cultivated.
That plus, selling out the gameday experience with Jumbotron ads and loud music, sterilized the atmosphere and destroyed its uniqueness. I love my college and my team, but even I don't want to put up with that stuff, and so I dropped by season tickets and stopped going to all but a handful of games.
And it's not like I'm the old man yelling at clouds, it's not like I'm in the minority. The proof is in the pudding-- attendance levels have been declining across most college football teams, for the past decade.