Hmm... I think that is because Argonians and Khajiits are "beasts", like they call'em. So, if you're a beast, even if you're evolved enough to speak and show a great sense of community, you don't have much of a culture. No culture = no weapons or armor I guess.
Argonians and Khajiits have their own armor. Every race does but the only way to see it is if the game takes place in their homeland.
Let's see...
Nord = Ancient Nord Armor. Dragon Armor.
Imperial = Imperial Armor (Normal, Captain, and Dragon)
Dunmer = Great Houses Armor (Remember those Spiky outfits?) Also, Ordinator Armor.
That we know for sure. As for the other races, we just have guesses like the Redguards have Scimitars.