There's probably not enough time in the movie to go through his life story and at the same time display his battles with Gatti while showing it's significance in the boxing world. As boxing fans, we want movies to show certain events but if it doesn't fit in with the storyline, people making a movie won't include it.

For example, when they made "Ali" with Will Smith and didn't include the rest of the trilogy with Frazier or him winning the title for a third time I understood because it was done properly in the movie.