Because they're based off of the FF12 variant of the race, and the outfit is a throw-back/bone to everyone whose been asking for Fran's outfit for years now.

Personally given their past history of more modest outfits and generally equal opportunity cheese when it does happen, I doubt that the majority of Viera NPC's will be wearing it. Likewise the men will likely be wearing something equally as cheese ridden if we ever get them, but that's a topic for another time.