TMP never expressed explicitly that all the uniforms were "spray 'n' wear", though. Just the robe the Ilia probe wore was even presented as such. As the other uniforms put on the appearance of pajamas, or worse, underwear in far too many instances, it's possible that Starfleet decided to move away from a style that at least looked flimsy and easily damaged, and toward one that, for all its formality, would stand up to constant wear.