How come in the NFL, teams that go far into the playoffs or to the Super Bowl, end up doing horrible the next?

next season and so on? For example, the 2002 Oakland Raiders, they made it all the way to the Super Bowl and lost, since then the Raiders have not been to the playoffs, they have been a disaster since the following 2003 season. The 2006 Chicago Bears, they made it all the way to the Super Bowl and lost to the Colts, and they did not make the playoffs in 2007 and last year 2008, and it looks like the Bears will not make it to the playoffs again this year. The Tennesse Titants went 13-3 last year and now look at them? they are 4-6 at the moment. Why is it so hard to keep a winning tradition alive in the NFL? but it's so much easier in high school and college football?