Why heavy naval guns moved into turrets

Why was the rotating turret developed; were fixed or casemate guns no longer good enough?

As guns became larger and heavier, turrets let fewer weapons cover wider arcs and bring multiple barrels to bear, freeing weight for machinery, armor, and fuel.

Deck-mounted guns could rotate across a much larger field of fire than the casemate weapons they replaced. Twin turrets multiplied the effect by bringing two guns onto a target without carrying separate weapons for every direction. The saved weight could then be invested elsewhere in the ship.