Most people care only about which game type they are sitting in, not so much who else is sitting in their room. For this, queue-based matchmaking is more efficient than room-based as is currently used.
So if 6 people want to play "4 player 10 vp fast hidden bank", they are guaranteed to enter the same queue and 4 of the get put into a room that starts immediately. The remaining 2 wait in the queue for 2 more to join. With room-based matchmaking right now what could happen is they end up splitting 3-2-1 or 2-2-1-1-1-1 into separate rooms all waiting for each other. Eventually people abandon their own rooms or wait for them to fill, but this increases wait time and/or effort required for matchmaking.
Private rooms, where people actually care about the players they're sitting with could continue to exist as is or we could make changes - password entry, invites, etc but that's for a future feature request.