Why destroyers used open mounts instead of turrets

Why did Royal Navy destroyers use open gun mounts rather than enclosed turrets?

Weight limits made a useful armoured turret impractical; an open-backed shield could provide splinter protection without trapping the crew in a thin-skinned enclosure.

A destroyer could enclose its guns only with armour too thin to provide meaningful protection. Concentrating the available steel in a shield offered better splinter resistance, allowed some projectiles to pass through, and gave the crew a route to escape a damaged position. A turret is an enclosed system that rotates with the gun, while a mount is attached to the deck behind a shield and can be turned by its crew. The open mount was a survivability and weight compromise rather than simple neglect of the gun crews.