I mean, when you consider all of the stories about knights in shining armor slaying dragons, it would make some sense to have Steel types hit dragons for super-effective damage.
Well, when you consider the already-existing type chart, then there's the fact that swords and armor are made of metal, and most dragons in those types of stories tend to breathe fire, and... yeah... --- Official Bride and Wife of Noire (of the Fire Emblem Awakening message board)
Also, as for Fairy-type (probably) being weak to Steel-type moves...maybe they took the Fae's inherent weakness to iron as an explanation...
I considered Fary's weakness to Steel and Poison being representative of technology and pollution encroaching on magic and the natural world. --- Socran's Razor: "Never attribute to massive stupidity by someone else, what could be explained by a tiny bit of stupidity on your own part."