I'll wager the film is more about the human spirit and perseverance during the Great Depression and boxing is the metaphor for this more than anything else... Even if the actual boxing scenes are an integral part of the story.
Films and stories like this get made on the universal appeal that everyone can or will indentify with James Braddock at one time in their life, regardless if they are male or female, Black or White, young or old, etc.
Sports movies are never about Sports. Even something as blatant as "The Rookie", or "Friday Night Lights" is about the human spirit and is basically about overcoming adversity in the midst of such dispair -- Be that real despair like the Great Depression or percieved despair like an unbeatable opponent as in "Miracle".
By the way... I hope James Braddock losing his title defense isn't a spoiler since I didn't know anything about him before this movie... Although, it is supposedly a widely known story and I suppose him losing it is more poetic in terms of it just being a symbol that inspires hope and not the thing that defines the man, correct?