That would be because Steel(Shouldn't) and Silver DOESN'T rust. Silver tarnishes.
Iron oxidizes very easily leading to the rust.....the addition of carbon when forging a blade leads to a much strtonger, more rust-resistant compound. Steel.
It should also be noted that steel blades were wiped with an oiled rag (or some equivalent method) to provide a barrier from the rust causing humidity & air.
I'd imagine that's why. And I'm not sure about the properties of fantasy materials like ebony and such, but since ebony isn't even metal, I'd guess it doesn't rust. And whatever things like Dwemer or elven are made out of, they probably don't rust easily either.
Of course, one might ask why you wouldn't see tarnished silver items, then, but it's worth noting that, if I recall, in Oblivion, rusty items aren't actually something you find often, mostly I think they existed to provide you with weak items in the starting dungeon that you'd quickly replace once you got even normal iron.