Why Submarine Deck Guns were Eliminated on German WWII U-boats

WWII German U-boats upgraded their armaments based on U-boat vs. aircraft engagement policy changes as the war progressed. The Germans implemented a fight back policy where U-boats would stay on the surface and fight back with allied aircraft. The May, 1943 fight back-order number 483 was issued by Admiral Carl Donitz. The heavy deck cannons were either replaced with smaller caliber anti-aircraft guns or eliminated entirely. The video will use historical records to outline the evolution of the German Submarine deck gun changes.
Text Correction at 13:29, I should have said unsustainable, not unattainable.