Because no .38 Super can shoot anything close to the 180 grain .357 Buffalo Bore, that's why.
And? The 9x19mm and .38 Special are poor sisters next to a .357 Magnum too and it doesn't stop people from buying revolvers in those cartridges.
As said, the popularity of 9mm revolvers currently is simply due to the plethora of inexpensive ammo available. There are other issues with the 9mm revolvers (crimp jump and requiring moon clips for extraction being the 2 biggest ones) that you can avoid completely by using a .38 Special.
Don't get me wrong, I own a 5" Colt Competition Model in .38 Super and think it's a great round. Especially in a semi-auto pistol that's as easy to shoot as a Government Model with good sights. It shoots flat and has very mild recoil.
But I don't see any advantages to clambering a revolver in it when the .38 Special and .357 Magnum are available. The rim isn't very big and I'm not sure you could rely on it to extract reliably in a double action revolver. A modern .38 Super like my Colt is setup to headspace on the cartridge case mouth, just like a .45 ACP or 9x19mm. The rim is just a vestige from the early days of smokeless powder and experiments with semi-auto pistols.
I also wonder if the question stems from the misunderstanding of the relationship of the .38 Super to the 9x19mm. They aren't even close to the same cartridge shape, lengthened or shortened.
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