Here's the thing: back on TOS, the female characters wore skirts, often quite short skirts. It was the late 1960s, after all.
By the late 1970s, Gene was getting a bit of flack from fans about that, and how it appeared sexist. So when TNG appeared, .... we saw that male officers sometimes wore skirts too. So it wasn't "sexist", it was just an option that the women on-screen in TOS overwhelmingly chose.
Then they disappeared, probably because they looked a bit silly and, as my mother once said, "It is impossible to do any kind of meaningful work in a skirt" (although I know some Scotsmen who would disagree).