Over-simplified
It's not easy. Moving wings or control surfaces forward and aft has many more consequences than moving "dead weight" as a door mechanism. For pax ops, the wing was/is in its optimum position. Now Airbus wanted to add some weight (the door) and it was better to put it aft of the wings and not forward. Thus the aircraft can be trimmed less nose-heavy and drag is reduced. In case of the B737 freighter the decision was different which proves that there are many factors more factors going into these decisions than I know of.