You can nitrite stainless steel, it's just not really worth it as it doesn't work nearly as well on SS as it does CM. You will get a higher surface hardness and more corrosion resistance just like with a CM barrel, but if CM gives you 100% improvement in those areas you'll only see maybe 20-40% of that with SS. As such the cost v benefit isn't nearly as good with SS as it is for CM so most don't bother.