I believe that's the gun that the Americans called "the Trench Sweeper" because of how effective it was when you made it into the enemy trench during a raid.
It, along with anything high capacity and rapid firing, are perfectly designed for filling long narrow corridors with hot lead in a short amount of time.
Shotguns were just easier to carry. Still are, mostly.
To be fair, history is filled with people calling shotguns unfair for one thing or another. Like the punt guns that would take out entire flocks of waterfowl at a time...