I'm going to take a guess that at the upper ages, especially 18u, that kids are deciding to play for school only. And why not?
I think it's hard for a lot of kids to do both, or have little enough interest in anything else to have the time to do both. Now if these club teams just played PAHL and a few tournaments, that's one thing. You can do 20 games, maybe 4 tournament weekends, 20 school games, and the practices to go with it if you wanted to. But add another 35-45 independent games to that, and now you're playing more than an NHL'er. And going to school, and trying to have a life in there on top of it. Makes it not very easy.
They come to the realization that chasing PAHL/Independant isn't worth what they might have to give up. Hockey is supposed to be a fun thing, maybe there are other things they enjoy, too. Whether it's spending time with friends, a part time job, spending a lot of time looking at colleges, wanting a few nights to play video games, or whatever else they could be doing instead of being at the rink 6 nights a week. Friends going out to the movies this weekend? Girlfriend wants to go to the school dance? Too bad, you have to drive to Philadelphia or Columbus to play a handful of games to count towards your My Hockey Rankings.
It's stressful, it consumes a big chunk of free time, you might get split up from your friends you like playing with, higher costs, etc. And it becomes increasingly apparent for just about everybody that there is no scholarship to Notre Dame or getting drafted to the OHL coming any time soon.
Or alternatively, you can play for school. Still be part of a team, still play good hockey, and actually get a crowd engaged in the game, other than half a dozen dads yelling over the glass at you. And unlike the independant teams, you're actually playing for something. You're at school half of your day, 5 days a week with your peers. It's more likely some of those kids you go to school with care more about talking about and cheering for their school team than the Pittsburgh Icemen, or the Allegheny Badgers did over the weekend.
I don't know if I'm right, but my guess is that is contributing to where these players have gone.