They just need to allow people to choose what game mode they want to play.
I think the reason they don't is to create the facade that the game is full of people queuing up to play Crucible. If the game modes were separated, queue times would get even longer and less popular game modes would become a ghost town.
English
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This is Bungie: you do not get to choose how you play. You will speed run every nightfall and you will play the mode of 4vs4 Crucible with locked perks and skill trees the way they want or you won't play.