Shotgun is gud since it actually kills xenos reliably, since they can very easily just run away from rifle fire. It makes sense for the shotgun to deal lots of damage, it's a shotgun. It's a reliable way for marines to single handedly take down xenos, while the rifles only really work if you have 2 or more firing at the same time.
It's not hard to survive a shotgun encounter doe, most if not all elder xenos survive a PB nowadays anyways.