Look to how much Rasmus Sandin has helped the Habs this series to understand why Ducharme doesn't trust Romanov right now. There have been many occasions this season where Romanov has tried to do too much and it cost him and the team. The coaching staff have looked at his games leading up the playoffs and decided, "He'll hurt more than help."
There is an argument to be made that he should be in regardless of the growing pains, but the coaching staff (and management) have decided now is not the time to throw him into the fire.