For decades, from as early as 2278 ("Cause and Effect" - TNG) to as late as 2349 ("Family" - TNG), Starfleet had the TWOK-style uniforms. They may have been modified later on but it was still basically the same uniform for over 70 years. Then, by 2354 ("Violations" - TNG), Starfleet has changed over to the early-TNG uniforms. Now, think about that for a moment: there's a major difference between those two uniforms and it seems like it would be a radical change to go straight from one to the other. What prompted going from a multi-piece uniform (including the jacket) that looks militaristic and probably too fomral for its own good to one-piece spandex jumpsuits? There must've been a change in Starfleet's outlook or philosophy around then. Or the times must have been changing. What would've been changing and what would the uniforms be reflecting as a result?