Large organizations with vast assets and many thousands of personnel do not make large institutional changes just for the sake of change, that's just not realistic.
And yet ST:TMP established that Starfleet vessels had "spray 'n' wear" clothing. See the Ilia Probe cool-down sequence. (The novelization and background press releases explained a lot of what we were seeing.) Surely uniform redesigns in this era are a simple matter of changing a computer command?
How often do we see airlines change all their flight attendants uniforms? Fashions change quickly and corporate uniforms can become dated if they don't keep pace.