[Pohl]

Because the far you get into the war the less you will use the deck gun, merchant ships begin to have guns and actually shot you but with a terrible aim, so you eventually stop using it around mid '42 (at least for me) I find the flak guns more efective or use a trained gunner to take out the merchant guns that way you can take your time to sink them or it could be to improve the speed while submerged