For some reason, Admiral Fisher liked extremely light secondary armaments

A quirk of Admiral Fisher was his desire for light secondary armaments. The Dreadnought was doomed to having 12pdrs, although in subsequent ships, the caliber was raised to 4inches. When 6inch guns returned in the Iron Duke and subsequent battleships and the Tiger, Admiral Fisher thought that this was a retrogressive step. He had fought to eliminate the 6inch gun secondary armament when he became First Sea Lord in 1904, and this defeated that move.

The new battlecruisers started after August 1914, the Renown class and the Courageous class went back to 4inch guns, although in the unwieldly triple mounts. After Admiral Fisher resigned over the Dardenelles fiasco, the new Admirals class battlecruisers were equipped with the sensible 5.5inch guns, as was the Furious.

In one sense, Admiral Fisher was "on to something", as it was generally realized that a lighter, dual-purpose gun was what was needed, by the mid-1930's. The Americans went down to 5inch guns while the British tried both 4.5inch and 5.25inch guns. The latter were a failure, as the guns and mounting were poor. They were not a good AA gun at all (at least according to my reading of D.K. Brown).