Why did Al Davis get rid of Jon Gruden after the 2001 season? why did he replace him with Bill Callahan?

Because the Oakland Raiders have not been nearly as successful since Jon Gruden left after the 2001 season, eventhough they made it to the Super Bowl in the 2002 season under new Head Coach Bill Callahan, they got killed in Super Bowl XXXVIII, since then, the Raiders have not been to the Playoffs, or Post-Season, etc. When the Raiders had Jon Gruden, they won 2 straight AFC West Division Titles in 2000 and 2001, now look at the Raiders?