@drewgriffithsA few points.
First; I agree that some people won't play if you can't go right to the NH. However; at that point the Memorial Cup play in realistically becomes if you aren't a top 5 draft pick, please start over. I would think "create a player" vs "be a pro" is a better fit for those folks. Create a player letting you build build an NHL ready player who actually belongs and won't be scratched 60 times.
Two, adding the feature doesn't really cost them anything, its all data and nothing more. The rosters, teams, logo's all exist for juniors and I think the other leagues as well because you can play them in free play.
Three the challenge of turning your 17 year old into a star is appealing to a fair number of players. Since the cost of implementing it is pretty minor why not do it to improve a mode that everyone finds pretty terrible.