I suspect that all of the TV networks were seeking shows that would best showcase the emerging color television market, too - I believe either RCA or Zenith actually used a Star Trek still image in a print ad. It's no coincidence that the show made extensive use of primary colors on the set. Even the uniforms were a bit bolder in color, compared with the ones used in the first pilot.
The first episodes that I saw - during the show's original run, - were on a black & white TV. By the time syndication re-runs began, a lot more of the viewing public had color TVs, which added to the surge in popularity.