Various Stainless Steel alloy's are available in reasonable quantities, each of which has a unique set of strength, machinability, magnetic and rust resistance among other characteristics.
Strength and machinability are the most important factors in my opinion as they significantly influence cost of manufacture and whether the mechanism will stay together when it goes boom.
All steel that I am aware of has a percentage of iron in its composition. Iron exposed to oxygen forms Iron Oxide otherwise known as rust.
A very minimum of care will prevent rust on SS guns and just a bit more care prevents it on non SS guns.
You will find that many if not all stainless steel guns have pins and springs made of conventional but high grade steel because Stainless is not suited to those functions.