Pump action shotguns seem like an appropriate weapon for the close quarters of trenches. Why didn't the other belligerent countries on the Western Front think of using them earlier in the war?
Partially it was because the only pump shotguns were American partially it was due to the American experience in the Philippines and thirdly Americans had experience with the use of coachguns and other civilian combat roles of the shotgun the highwayman in england ceased to exist before 1800