I found myself grinning at how frequently Mr. Mack drew attention to "the new miniskirt uniforms for female officers" in the book, going as far as making Uhura feel uncomfortable and self-conscious in Kirk's presence due to wearing one. Yet the Whyyyyyyyy would they make uniforms like that?
question is never actually asked by anyone
To be fair, there's really no rational justification for why the 23rd Century Starfleet would create such blatantly sexist uniforms, especially given the embrace of sexual egalitarianism that our society has made in real life in the decades since TOS was produced. The closest you can come would be to say that the miniskirts are optional and to then depict plenty of female officers still wearing trousers.