It just seems that who ever designed the legionairs and other 'guard' NPCs didn't bother to make them more diverse. Perhaps they figured a clean image of how a guard is supposed to look is better than having them be diverse. By keeping it simple they can easily paint a picture of the two sides. However they aren't consistent as important NPCs are diverse. To me it also seems weird that all the guards are the same almost faceless, characterless, soulless objects. It would definatly have been better if the Legion had more races in it. Orcs in particulary should be more prominant but there could also be a Khajiit and a Dunmer from time to time. And on the Stormcloak side it would be weird to have Elves, but there could be Imperials, Bretons and Redguards in their rank. Just humans who feel they must fight against the Thalmor.
Khajiiti in the legion would be considered traitors to their kind by their kin. The Khajiit are willing (unsuspecting) vassals to the Thalmor. The Redgard have seceded because the empire back-stabbed them to save it's own skin, so you'll be hard pressed to find a single Redgard who wants to fight in the legion. Instead the Redgard have formed their own military might which kicked Thalmor ass. The Orcs have other interests atm, as do most of the Dunmer. The Bretons.. They are more interested in the mages guild then in the legions, though the mages guild could very well pick up arms alongside the legions. So, what the Imperial Legion is (and should be!) left with is mostly Imperial and Nord recruits.


(Thank Obsidian for "tim", Obsidian because they first included it in New Vegas
)
