Because of what he has done over the years going back to the White Sox with LaMarr Hoyt, then Oakland he turned Dave Stewart into a 20 game winner, helped turn Bob Welch around into a Cy Young winner, helped Dennis Eckersley revive his career and make him into a Hall of Famer, and now with the Cardinals- Developing Chris Carpenter into a Cy Young winner, the work that he has done with Adam Wainwright and now Garcia, getting the most from guys like Jeff Weaver and Jeff Suppan, and now it looks like he is reviving the careeer of Brad Penny.
I think Leo Mazzone was very good- but he had three Hall of Fame pitchers that were very special and he should get some credit for that, however the Braves bullpen was often average, and when Mazzone left for Baltimore he really didn't do very much and was gone in a very short time.