It's mainly because if you say "the movie '9' from 2009" or "the movie 'Nine' from 2009" it sounds the same.
It's pretty common to use years to distinguish between movies with the same name, which is impossible in this case.
12 Angry Men is my favorite movie, probably. It's sometimes referred to as "12 Angry Men (1957)" because there have been other adaptations of it. This works, though, because there aren't any other notable adaptations of it from that same year.