I'm sure there are valid reasons. But it's similar to not allowing the import of NCAA rookies in madden. (Not that I did that.) People want to play a certain way. If you had a full 30 user GM CCM you are talking about 2,460 games to be played. There has to be a better way. Sure you could sim games, but imagine if X team loses (star player) during a simmed game. Guys get mad, guys leave the CCM. I'm sure they had their reasons, but to say. No short season option, but hey you can sim games, to me comes off as wildly arrogant and indifferent to customer needs.
But I'm not a big NHL gamer so I can't claim to know how the masses feel.