Well, even on men, less armor meant that you valued mobility more than protection. Skilled fighters would just wear plates on the places that were most likely to get hit while leaving the rest of the body unencumbered.

A chainmail HALTER TOP would actually make a lot of sense, by this strategy. It would protect the heart, the leather underlayer would protect the breasts, and it would leave the waist and shoulders unrestricted for greater freedom of motion, AND it would still satisfy people who want to see a lot of skin on the character.