Typically movies spun-off from another medium would simply be subtitled "The Movie", much like Superman was advertised as such a year prior. I honestly can't recall many movies that use "The Motion Picture" as a subtitle prior to Trek. An IMDb search isn't really helping either.
Well, why wouldn't they? "Motion picture" is the more formal term; "movie" is (or was originally) a slang shorthand for "moving picture." Maybe the distinction is gone by now, but when I was young, I had the impression that "movie" was still considered slang. So a film that wanted to appear reputable and classy, as TMP did, would prefer to use "The Motion Picture." Or maybe "The Film."