TMP never expressed explicitly that all the uniforms were "spray 'n' wear", though.
Because the Wild West movies didn't explain how rifles worked when they fired them either.
The "spray 'n' wear concept" was described in publicity sheets of the day, the "Making of..." book and the novelization. None of it canon, of course, but how often was the mechanics of a rifle explained in westerns?
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.
And seemingly impossible to put on, or remove, by oneself.