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One thing that has to be remembered is that since ships were limited by treaty, this meant that weapons were also indirectly limited by treaty as well. The quad 1.1 weighed less then half of what a quad 40mm weighed. Until the treaties were abandoned there was little reason to develop heavier more powerful weapons if they could not be put on the treaty sized ships. And even with the treaty abandoned you still had treaty sized ship which had to be stripped of non-priority material and overloaded to fit the more powerful weapons. On the US treaty DD's they usually had to lose a 5 inch gun or a torpedo mount or both to fit the 40mm and 20mm weapons

The British 5.25 in my opinion suffered from the same problem, in order to fit it onto treaty BB's they had to shave down the size of the mount and limit training and loading machinery and this limited it's effectiveness against aircraft. It was only when they built the unlimited Vanguard that they developed a larger heaver mount that was successful.

So if you tried to sell the US Navy a 40mm or 20mm mount in 1938 they would look at it and say, looks good, but it won't fit our treaty limited ships without overloading them since we are pushing the limits even with .50 cal and 1.1 inch. This even went into the war years since most of the war built ships were based off treaty ships. It was not until the Fletcher for example that treaty thinking was abandoned for DD's

I don't buy that excuse.

Aircraft performance improved by leaps and bounds between WWI and WWII.

And there was no treaty limits on aircraft.

Yet for example destroyers were limited to either 1,500 or 1,850 tons and as seen by what was built they were all overweight when post treaty weapons were installed. Look at all the later DD which were built after the treaty designs were abandoned, they were all much larger

If you look at other ship based AA weapons available in 1940 it appears that most were far from perfect. American, British, German and Japanese naval AA systems all had problems during this era.

And they were all picked based on fitting on treaty sized ships.

It was not the only reason but I think a big factor was that nobody was going to pick up the 40mm quad gun to develop in 1938 when they already knew that it would not fit on their ships. Imagine the uproar in peacetime if they picked the gun and then had to strip off the boats, a five inch gun, a torpedo mount and reduce stabilty on a Farragut class destroyer just to fit the new guns.

Even late in WW2 the US navy developed the twin 20mm, not because it was more effective, but because it would save weight and cut down the number of crew since the war build ships were overweight and overcroweded