Why the Royal Navy fielded several medium gun calibers

Why did the Royal Navy operate so many overlapping medium and small gun calibers?

They came from different roles and periods, and the planned shift to a 4.5-inch standard required years that wartime demand did not allow.

The overlapping calibers came from weapons developed for different periods and roles. The 4.7-inch gun remained a destroyer standard, the 4-inch weapon served anti-aircraft needs, and the 5.25-inch offered a heavier dual-purpose option. The 4.5-inch gun was intended to combine requirements and become the long-term standard, but converting a huge fleet could not happen quickly. Admiral Henderson expected a process begun in 1936 to take until roughly 1944, so wartime necessity kept several mature lines in service.