S&W originally used stainless steel hammers and triggers on their stainless revolvers. Customers complained about the resulting bad trigger-pulls, so S&W went to flash-chrome-plated carbon steel for the hammers and triggers.
The good trigger-pulls then returned to S&W revolvers.
Here is a related post from another thread:
Let me start by saying, "I am not a metallurgist"
Different gun parts require different properties: hardness, strength, light weight, etc. The designer should specify materials and heat treatments (with the advice of the metallurgist) that meet the requirements of the firearm.
On a stainless steel gun, one of the design requirements is corrosion resistance. Some parts are not best made from stainless steel - (S & W used to make stainless steel triggers and hammers for their stainless guns, but these parts were not meeting the requirements, and they changed these parts to chrome-plated carbon steel. Also, some early stainless S & W revolvers had stainless steel fixed front and adjustable rear sights!)
Designing any product is a balancing act - the product has to be producible, resonably priced, desireable to the consumer, and do the job for which it was intended.
People must want stainless steel guns, or the manufacturers would not make them!
-Mk.IV