Aside from the duds problems that plaguered the US torpedoes until late 1943, the german ones had much more powerful explosive loads, so they did effectively same or even more damage with less torpedoes.
Because the americans expected their magnetic torpedoes to work well, they thought they could break the back of an enemy vessel with just one torpedo fitted with a lighter explosive load, and that design limitation was carried through the war in the MK14 torpedoes when they switched to impact detonator:hmm: