Well iron and steel are in real life the best commonly available materials for weapons and armor. DF reflects this in a general way.
Actually, Iron is a somewhat inferior metal for making weapons and armor IRL compared to Bronze and steel... the reason that we "graduated" from the bronze age to the iron age is that the metals needed to make bronze alloy became too rare, and because Iron is easier to work and abundant on the surface of the earth.
So glad someone else mentioned this very common misconception. Not only is bronze harder than cast iron (though comparable to tempered iron), but it also will not corrode away. Once a piece of bronze develops that green patina, the metal beneath is protected from the elements.
Bronze > Iron