The Arizona Diamondbacks won the World Series against the New York Yankees the same year Jason Newsted decided to stop playing bass for Metallica. It was a long time ago, but it was never really mentioned why he left. Finally in an interview with ScuzzTV, Jason revealed his side of the story on why he left Metallica. He claims that Metallica's management was really enthusiastic about his other band, Echobrain, and had a desire to work with them. Jason also said he listened to the record and called it a "diamond." Word had gotten back to James Hetfield and apparently he was not pleased with the situation. Apparently he did not want Jason to being doing another band and told him, "Other arrangements can be made..." Jason then decided to leave Metallica and go his own way. This is Jason's side of the story. The other members of Metallica have not mentioned a reason why Jason left the band.
The interview where Jason tells his side of the story is in the video below: