With the American domestics, its because all the frequent flyers are flying with large carry-on's and large personal items. Combined with all the non-frequent f

With the American domestics, its because all the frequent flyers are flying with large carry-on's and large personal items. Combined with all the non-frequent flyers trying to avoid baggage fees by packing everything in a large carry on. If you don't get on fairly early after them, its likely you won't have a place to put your carry on, or it will end up being in the back of the plane/whatever forcing you to wait longer to get off.