It really doesnt.
Skyrim has been a part of the Empire for a very long time.
Vvardenfell was only open for Imperial colonisation for 200 years, yet Morrowind had more racial diversity than Skyrim.
This is a design choice, probably to make the game more accessible or to get more 'Conan' in there, either way I think its a terrible shame. The diversity of races was a strong suit of TES.
Except that Khajiit are actually outlawed from the cities. Why the heck would a Khajiit come to Skyrim, the coldest and possibly most inhospitable province, when they know that they'd get turned down at the gates of every city?
As far as Argonians go, we're a very long ways away from Black Marsh, plus the fact that the climate can't be very good for a reptilian race like the Argonians, plus Morrowind's Argonian population was inflated by the the fact that many of them were there as slaves so comparing it to Morrowind is rather unfair.
My guess is that we'll see a lot more racial diversity in the expansion, similar to how Shivering Isles spiced up Oblivion.