I guess, if anything, the dragonborn is a hereditary thing, so it makes sense that a human would get it. I mean, the first dragonborn was a man, then you got Talos who was also dragonborn, both of them northmen (I think). So it'd make sense that Talos's offsprings just passed it on down and the dragon soul just happened to land in someone years later. Highly unlikely that that would be an elf, uno.

This is just a theory that sprang up on me like right now, so I don't know.