The answer is actually a bit simpler than that; Sylvanas got a new model in Wrath of the Lich King based on her Warcraft 3 look. This was back when very few NPCs got unique models. Instead of making different models for all 3, Blizzard just recycled Sylvanas plate bikini for them. Ysera didn't even get a unique dragon model until late in the next expansion.
Unlike the other two, Sylvanas is still a major story player and her model got updated to make a little more sense for a former ranger general (her original model was a generic Night Elf):
Now, as to why they decided to not give Alexstraza Sylvanas' pants I don't know.