Stainless steel is stainless because it is made from Teflon and everyone knows nothing sticks to Teflon (except frying pans). Because nothing sticks to it then nothing can discolour it or stain it. Because nothing can stain it well, it is rather "stainless."
Now, this doesn't make sense, I hear you say. If it is made of Teflon then why not just call it Teflon instead of stainless steel? Well stainless steel has been a misnomer for ages in that it wasn't really stainless, oh, and it wasn't steel either. Stainless steel used to be made of copper and as everyone knows, copper turns green after a while, so this choice of name was very bad in so many ways.
When manufacturers started making stainless steel from Teflon and noticed that it doesn't stain, then the name stainless steel stuck.