Everyone explained the various reasons. I don't really buy into it. .357 magnum is my favorite revolver caliber, was made for the revolver. I'm not a big, slow kinda guy so I don't really like those .45 ACP revolvers. If I'm going to carry a .45ACP, I'll carry my P90. .45ACP revolvers are great for revolver action sports, though, especially for those that require momentum like pins or pepper poppers. I had a 1917 Smith for a while with that in mind, but it was sorta a rat and I sold it. I'd have kept it if it'd fired cast lead bullets, but the shallow rifling wouldn't stabilize anything without a jacket. .45ACP is so fat, small J frames are impossible. 9mm is such a high pressure cartridge, they're usually in heavier built guns than are .38s and even .357s. A nine could be put in a titanium gun that is chambered for .357, but they aren't. To me, I shoot the cartridge that was designed for the guns, .38 or .357. I'll leave the rimless stuff for auto pistols. I mean, why would I wanna carry a 9 in revolver when I can carry it in my Kel Tec P11, 14 ounces unloaded, up to 13 rounds capacity, and smaller dimensions than a revolver? The gun is very easy to shoot for a DA guy, smooth, long DA. I think of it as a little square revolver with the ultimate speedloader.
One thing about moon clipped guns, tactical reloads are impossible, at least the topping up kind. I carry at least one bianchi speed strip with me when I'm totin' my revolvers just for topping up if I were to need to while behind cover in a lull. Speed loaders are fast enough IMHO for speed reloads. I'll never be as fast, even with moon clips, as I am with an auto, but I don't dwell much on reloads. In your standard self defense scenario, a few shots fired is all I can imagine, not being an LEO. But, I carry reloads for the same reason I carry the gun, just in case.