The Bismark was also attacked by biplanes with defective torpedos (thankfully, that saved HMS Sheffield). Basically only two torpedos even hit the german battleship.

Presumably because the british torpedos were so awful, Tirpitz was attacked with regular bombs, which meant they were using the worst method of sinking a ship, from the top down, and so it didn't do much until they whipped out the ultra heavy ones. And it's not like the attacks were going poorly, Tirpitz was taking the hits because it could not kill the planes.