Why British destroyers kept the 21-inch torpedo

Why did the Royal Navy develop a 24-inch torpedo but not fit it to destroyers?

Its added weight was not worth the improvement under treaty limits, so Britain pursued better 21-inch weapons instead.

A 24-inch torpedo imposed too much weapon and handling weight for its advantage over a 21-inch weapon under treaty limits. Torpex offered more explosive effect within the smaller weapon. Smaller treaty fleets also presented fewer capital-ship targets, so launching enough torpedoes to secure a hit became especially important. Eight or ten 21-inch weapons could offer better chances than six 24-inch weapons. The weight comparison also had to include the gear needed to load and handle the larger torpedoes. Torpedo-storage positions and hauling rings aboard Tribal-class destroyers suggest that British crews manually reloaded torpedoes at sea, though this remains an inference from those fittings.