Because it's much, much easier to carry a compact 9mm. 357 Sig is effective but really, really loud. There are no excellent compact 10mm pistols that I've seen. (I don't consider Glocks to be excellent, and I would not want to shoot the shorty 10mm, anyway.)
All handguns are a compromise. A modern thin 9mm with enough capacity and good defense ammo stop be more than enough RI not be outgunned in most cases.