"Rocky" was released in 76. My dad went to watch it way back in 1976 and, a couple of days ago, I was talking to him and he told me something curious: In the movie, Rocky is just an ordinary man, who has a normal job, who goes through your usual underdog story and the movie is well written to the point where you can relate to him and, by the end, you're rooting for him against Apollo Creed. The fact that he is not a superchampion, a famous fighter and all, just a guy who trained really hard and has his own personal struggles help on this.
But funny enough, when the movie was released in Brazil( where i'm from), the Movie got a subtitle added to it: Rocky - Um Lutador, which translates to Rocky - A Fighter. My dad told me that, when they saw the trailers and the title, that " A fighter" implied exactly what the movie tells you: He ain't THE fighter, he's just a normal mundane worker, you can relate to him just by the title of the movie. Sure, "Rocky - The Fighter" can sound more cinematic and epic, but "A Fighter" captures the spirit of the movie