It would have just been STAR TREK except they worried that people wouldn't realize it was a movie and not just the TV show. The title was the right one for the time. The proposed subtitles aren't improving it, even if some are amusing.
It's probably worth noting that the first Christopher Reeve "Superman" movie, which came out around the same time, went the same route: Superman: The Motion Picture.
It makes it sound like more of an event, than just a movie based on old tv show or comic book.