It's all about money. The contracts the networks had signed with the NFL expired after this season, so they renegotiated at the beginning of the season.

Sunday Night Football is moving to ESPN because ABC (a sister station to ESPN as both are owned by Disney) wants to open its Monday Night Lineup to other "regular" TV shows, especially with the success of "Boston Legal", "Gray's Anatomy" and "Desperate Housewives."

Starting next year, NBC is going to broadcast the Sunday Night Football games since they haven't had any successful primetime Sunday shows for quite some time.