SS doesn't have the strength of a chromemoly steel at high temps. Notice that all of the stainless barrels are fairly heavy. If you were to use stainless in an M4 profile barrel with a suppressor, for example, you probably wouldn't make it through a whole BetaC mag on full-auto before the barrel burst. It would fail either just forward of the chamber or at the "M4" stepdown.
SS also doesn't resist throat erosion as well, so while the barrel can give decent accuracy after full-auto use, you won't get the accuracy that most stainless barrels are noted for.
Like a lot of materials science, changing steels for barrels isn't a 1-to-1 swap. To get something, you usually have to give up something. In come cases, the only thing you give up is price (i.e., higher cost), but usually there's some kind of performance penalty somewhere.
-Troy