There are some culture-specific weapons, such as the Stygian Spear, that are in addition to the vanilla versions of similar weapons. That’s why there are more than one weapon of the same type in some metals. They’re for some extra flavor that connects the Exiled Lands to the rest of the world of Conan. It’s possible that some more weapons will be added in the future, but some weapons, such as maces and warhammers, just don’t have a good in-world “cultural” equivalent, so all we get are the vanilla versions.
I don’t understand why there’s a katana in the game at all. Yamatai isn’t based on Robert E. Howard’s stories, it’s a Marvel Comics creation. (Well, I guess I understand why it exists - geeks are often fans of Japanese stuff, so it’s easy money for Funcom.)
But if there has to be a katana, it’d make more sense if the lower level version was made of steel instead of iron, considering that the item’s description refers to the steel folding techniques… That said, there’s no real reason why a katana couldn’t be made with any available metal - but that logic of course applies to all weapon types.