Sports films typically suffer from predictability - we KNOW the boxer will win in the end, the team will triumph, the jockey will cross the finish line first, the underdogs will come out on top. Anything less would not fly. This predictability defines the divide between rooting for sports and watching sports movies.
When dealing with a game whose outcome is already known, a film audience can't be made to care whether the heroes win or lose; instead, the focus of the film must be on whether the heroes transform themselves (or others).
The key to "Moneyball"'s success is following whether Billy Beane's efforts change the way the game is played, or whether the "bad guys" will rule at the end of a most unorthodox season. Even more intriguing is whether Beane can somehow exorcise whatever demons he may have regarding his failed playing career.