Why Bismarck's forward turrets were disabled

Were Bismarck's forward turrets poorly armored, or did Rodney obtain a lucky hit?

The armor was substantial, but a close-range 16-inch hit could still overwhelm it.

Hit placement and weapon power both explain the result. The British 16-inch gun was a First World War design, but remained thoroughly capable of its intended task: defeating a German capital ship. Rodney was firing at unusually close range, where even substantial turret armor offered limited protection. Armor figures therefore mean little without the attacking gun, impact geometry, and range.