Why isn't this cartridge more popular?
Popular with alot of state and federal folks.
But most civilians are not 'into' guns all that much and thus they stick with 9mm, .38 Spl., and to a lessor extent .40 and .45.
The .357 Sig is my CCW cartridge of choice as for semi-autos. With loads from Double Tap you get true .357 Magnum performance even from the small Glock 33.
I figured why pay $$ for +p+ 9mm that is hard to find when I can get good .357 Sig JHP ammo from most shops and it's more powerful than +p+ 9mm.
But again, most citizens are not into guns. Hence even the .380 and .32 are still popular!