Lots of speculation, but there is only one REAL reason for no .357 sig rifles: PCC's are all blowback and it's difficult to safely time a blowback action for such a high pressure cartridge. MAP for .357 sig is 40,000 PSI, higher than even the 10mm Auto.
Any carbine chambered for a pistol cartridge is arguably just an oversize pistol...You carry a small rifle that can't defeat body armor, or reach out too far.
Not too many pistols out there have the 100-150 yard effective range a PCC does. As a civilian, most people are not going to want or need to shoot further than that so they decide to go for the many advantages that PCC's have.
1. Straight wall pistol cartridges are easy to reload with carbide dies and the brass lasts almost forever.
2. PCC's easily handle cast or plated bullets. I never did, but several local reloaders made some good coin selling their cast reloads during 2008.
3. During the 2008 shortage, pistol ammo was much more available than rifle. Your .223 or 7.62 rifle doesn't do much good when you've got no ammo.
4. Ammo interchangeability with ones handgun. Not everyone owns 7 dozen guns and ammo for all of them. A PCC fills a gap between the 22 rimfire and the deer rifle while not requiring another caliber be stocked.
* Lots of people oooh and aaah about carbines for 9mm and 40 S&W, but few pull out their wallets.
Hi-Point sells their 9mm, 40S&W, and 45 ACP carbines as fast as they can make them. There is a strong market, you just have to be able to keep it in the price range.