I think because
Charlie is still definitely in use for boys, the usage for girls shouldn't be too much of an issue. It isn't like
Meredith or
Aubrey or
Avery where the usage for males is minimal so it is easier for them to become feminine names. The nn thing annoys me.
Charlie and
Charles have such different feels to them.
Charlie is such a 'nice guy' name and
Charles is so pompous sounding... :-(