I'm surprised to hear you say that the OHL and the Q don't share this problem. Not that I doubt it, I'm just a little surprised. Certainly, the Dub has several smaller cities with teams. I don't know if they are undesirable or not. They may struggle to compete financially which doesn't help. I think the players who graduate from those teams are usually pretty positive about their experience. But that still may not make some of them attractive to the players coming after. Travel is an issue also. Those are some long bus rides.
But my assumption was simply that the US scholarship route was increasingly attractive to young players. I can remember a time when few NHL'ers came out of the NCAA. Almost any Canadian kid who was even remotely good could get a scholarship if that is what he wanted. But times have changed. As the level of play improved in NCAA it became more attractive as a path to the NHL, leading to further improvement. Its a process.