Actually, there were laods of women in trousers, as well as in the minidress uniform with long sleeves instead of cap sleeves. They were mostly background performers, but there's even one female officer on the Enterprise bridge in a red Ops tunic and trousers. I say Uhura had to wear the minidress in the first film because of the iconic image of Nichelle in her uniform on the series. but in the next film, it would make more sense for her to have trousers on a Landing party. But clearly there were several uniform options for female Starfleet officers and crewmen, cos you see them all the way through the film, in every department aboard Enterprise.