Because our medical staff is overseen by Dr. Mulder, a relic of the past who doesn't seem to understand concussions. How much actual impact he has on the every day medical events in the organization, or whether he's there mostly to overview (or to be a figurehead, in a way), I don't think we'll ever know. But whenever there's a very serious injury at the Bell Center, they always bring him on the ice to take a look.
In recent years, there's been a bunch of times when players on the Habs got hit in the head, got up on wobbly legs, and were allowed to return to get game, only for the team to announce the next day that the player has a concussion. "Well, the player showed no signs of a concussion last night and said he was fine"