Mount weight kept the 4.5-inch gun off the Tribal class

Why was the 4.5-inch gun not available for the Tribal-class destroyers?

The upper-deck dual-purpose mounting was not ready and was too heavy for the design.

The gun existed, but the mounting needed for destroyer use was too heavy and still under development. Attempts to reduce weight repeated a familiar pattern seen in British turret design: weight was removed, some had to be restored to make the machinery work, and the resulting mount still fell short of the original concept.