The big thing that sticks out at me is that if they do that, then what happens with the next flight that plane is supposed to make? They'll have to get catering, baggage, cleaning crews, etc., over to it, then bus the pax for the next flight over, as well.

In the alternative, if they deboard the incoming people and their bags, then move the plane over to the gate for the next flight, that means that while they're offloading people and their luggage they can't be doing anything else, like having catering or cleaning crews get a start.