To the victor go the spoils, although sometimes the victor gets spoiled. The only championship teams to repeat the next season since 1979 were the 1998-2000 dynasty New York Yankees and the 1992-1993 Toronto Blue Jays. A few more champions over that time went on to at least make the World Series the next season - the 2009 Phillies, the 2001 Yankees, the 1996 Braves, the 1990 Athletics. On average though, World Champions sport just a .538 winning percentage the following season in the Wild Card era, according to Ben Lindbergh at Grantland.
The main culprit for the decline seems to be that it is hard to keep the band together.