First because it was about actual ballplayers (obviously several baseball movies are about players who never existed, such as Ray Hobbs in The Natural or Crash Davis in Bull Durham. 8MO captured the time period extremely well and to me the casting was excellent - even though the actors bore little resemblance to the ball players they portrayed, with the exception of Michael Rooker (right) as Chick Gandil (left).
Charlie Sheen didn't look like Happy Felsch.
Note that the producers of Moneyball did not go out of their way to ensure that Brad Pitt resembled Billy Beane. Nor does Phillip Seymour Hoffman resemble the very fit Art Howe. (But then Gary Cooper, who played Lou Gehrig in The Pride of the Yankees, was no where near as muscular as the Iron Horse.)
It didn't matter too much whether the actors looked like the gamblers they portrayed. Apart from "Sleepy" Burns and Billy Maharg who testified at the trial the public never saw Arnold Rothstein and Abe Attell, the big money men behind the fix. Unlike the players, who received only a fraction of what Rothstein and Attell won on the Series, the gamblers got immunity from prosecution and never had to appear in a court.
Probably the most memorable scene in the film is the one in which a young boy outside the courtroom says to Jackson, "Say it ain't so Joe, say it ain't so!" In a 1948 interview Jackson absolutely denied that any such conversation ever took place.