I would assume it has to do with presence.
A imperial guard of the land is much more threatening and enforcing than...a cat? Just doesn't make for a good presence as a guard.
I think I'd actually find a Nord or Orc more threatening, to be fair, Morrowind shows us a few orcs in the Legions, but Cyrodiil seems to lack them.
In any case, I'd never find Imperials very threatening, though I suppose they're better than Bosmer, I mean, they're so small, and those voices, I don't care if you give them full Daedric armor and have them carry Umbra, there's no way they're scaring anyone.
But that variant rarely ever leaves Elsweyr.
Which doesn't make any bit of sense, considering that the different types of Khajiit are determined by the position of the moons when their born, so even if they don't leave elsewhere, there's no reason why they wouldn't be born outside of it.
Cause it's an Imperial province and therefore defended by the legion? Is the legion strictly Imperial only?
I believe the answer is no. In Morrowind, any race could join the Legion, and to show that this is not just gameplay mechanics, there are also NPCs in the legion of other races, such as the orcs I mentioned. I guess is that Bethesda got lazy and just found it easier to make them all male humans, or maybe, Imperials are more likely to join the Imperial Legion being as they'll be serving their own Empire and all that.