Lux's boobplate adds to her youthfulness. Kind of like a training bra vibe. Don't hardly even notice that Leona has boobplate.
Leona will probably get a full breastplate like the one you see in the "A New Dawn" cinematic. As for Lux, her armor is more for fashion than function (it's not really meant to save her from a sword blow), so I see no reason to change it.
