Uhura's outfit seems to be an optional variant as there were also female personnel wearing full sleeves and trousers as well.
As we saw other Starfleet outfits that didn't feature rank insignia (the space-diving outfits and Kirk's cold weather gear), rank insignia may not be that
terribly important to be on every
Starfleet outfit. Even in TOS, McCoy and M'Benga's surgeon tunics didn't have rank insignia, neither did Nurse Chapel's uniform, or the formal uniforms (although some braiding along the front was used to distinguish senior and flag officers, IIRC).