The uniforms of TMP were non-martial, and Roddenberry wanted it that way- Starfleet was about exploration, not conquest. Meyer and company went with military uniforms- playing to base ideas.
Have you seen the "uniforms of the future" that used to be showcased at EPCOT, Disney World, Florida, in the 70s? Based on extensive research and extrapolation, they look intriguingly like the Starfleet uniforms Robert Fletcher ended up with for TMP a few years later.