Or it could just be because Bethesda wanted to add a bit more attractiveness to female Argonians. Seriously, I think you guys are over thinking this.
Well, they have been around since TES: Arena and if you look at the woman on the cover of the game, it is hardly surprising. :P
While they can lay eggs, most of the time, at least outside of Black Marsh, they don't. They're ovoviviparous.
To help distinguish the genders in a way understandable to the average person and to make them appear more Human. How many games/movies/manga/etc. can you think of, that has a bipedal race as the protagonist that has no visible gender differences?
The Elcor aren't Bipedal. The Slarians are Bipedal, but do have visible gender differences, just not as obvious to people who are used more Human-like characteristics.
I also deleted the post because neither one are protagonist races.
NECROing
MANNIMARCO SHOULD RULE
There is no in-universe explanation. Bethesda was simply lazy. They could use the same model for the body and clothing, they just had to add a new face model a tail model and different textures. I for one prefer the less humanoid design from Morrowind. Divinity: The Original Sin could also create sexually dimorph designs for lizard people without breasts.
Due to the power the Hists gave to the argonians, they are able to both lay eggs and give birth like mammals in certain conditions. When they are pregnant in Argonia, they lay eggs in underground pools of hist sap, while in other parts of tamriel/nirn/anywhere else, where there is no hist sap, they don’t lay eggs but instead feed their children hist sap the same way mammals in real life feed their children with milk.