
There is no such 'why', just a conceptional 'that's why'. Likewise you might ask why Jedi knights and Sith Lords are multi-racial. Being dragonborn has nothing to do with the race you choose but with playing 'the chosen one' of the devs, the force or whatever you like to call the given concept, a concept to fulfill a single prefab purpose. Just take it or leave it. There is no primus inter pares, no first race among equal. Don't read something into the Lore, okay? Instead, do your frakkin job on the grid as good as you can. And don't ask me to illustrate the racial diversification of the dragonborn, I'm not on Bethesda's payroll and the screenshot section is elsewhere ha.
Edit: Indeed, my prologue above is written in Jamaican Patois, the language of Reggae, Soca and more... just for fun, as Aurora would have said. I remember...
Nords, Imperials, Bretons.
Edit: This topic is getting old. Basically Akatosh/Shezarr are human gods so it'd make sense for Dragonborn to be human. Auriel, Alkosh, Alduin =/= Akatosh/Shezarr.
Worst example ever when he's clearly not normal. If he's not an aspect of a god, Thalmor aren't pompous.
Still, he's a Redguard and there is probably a reason he's a Redguard.
I'd go with Altmer, just to annoy those "SKYRIM IS FOR THE NORDS!!!!11!!!" types.