Why deck armor grew more important after 1943

Did growing deck armor reflect aircraft threats more than battleship gunfire?

Both mattered: radar fire control improved the prospect of plunging fire, while aircraft were an increasingly significant threat, including aircraft carrying heavy weapons.

Aircraft were a major reason for better horizontal protection, though they were not the only one. Improved radar fire made more accurate long-range trajectories relevant as well. Deck armor therefore answered an evolving combination of aerial and surface-delivered threats. Heavier horizontal protection had to account for both aircraft weapons and plunging gunfire.