Hm... I think the "medieval" nature of stories has more to do with the historical tradition of literary romances like "Silence" and "Le Morte de Arthur".
However, beginning about the time of Conan and Tolkein, things tended to become all a series of events that exist in a time beyond time, and a place beyond place.
Medieval military tech and tactics may be the norm, but Conan is not describable as "Medieval" any more than Earthsea.
I think the signs of manhood carried over from the medieval romances. Iron tech became a convenient artistic shorthand and the visceral style of combat was right against the edge of modern warfare - thus, tactics were still pretty close to what was familiar.
But, I don't think it's actually "Medieval". I think it's just a set of convenient symbols. Really, each fantasy novel is the modern world with different signposts, and different technological influences. If anything, it's science fiction with swords instead of lasers and elves instead of aliens.
If everyone says - and we do - that SF is just fantasy, the reverse is also true.