Arguably, because they were on a naval(? not military because Starfleet isn’t, but…) vessel in professional roles. If women had been in the air force in the 60s I doubt they’d have been in mini skirts.
Not miniskirts but:
https://myairmanmuseum.org/wp-content/uploads/2022/10/Nellis...
Anyway in the 1960s the navy was making movies like this:
https://youtu.be/rUKcHe0-m_I?t=234
Not miniskirts but:
https://myairmanmuseum.org/wp-content/uploads/2022/10/Nellis...
Anyway in the 1960s the navy was making movies like this:
https://youtu.be/rUKcHe0-m_I?t=234