To elaborate a little: most NPC's don't have to go through all the animations that the player characters do, chances are many of those uniform pieces we can't use would distort horribly or clip far worse than the pieces we currently have do. many of them probably aren't set up to be used in conjunction with other parts too so there would be even more problems there making sure they looked right with parts you normally don't see with them.
Also there is little variation to the NPCs in terms of size and bulk so many of those costumes might only work with specific "standard NPC shapes" and might otherwise clip horribly.
The problems that exist (ed?) with the Generations/Voyager Communicator on Female Starfleet officers would be an example of this (They disappear (ed?) when you pulled the breast size bar to the right too much)
I am not sure whether this bug has been corrected, hence the (ed?).