BlueJ1,
As I understand it, when The NFL first wanted two teams to play on Thanksgiving day no teams wanted to do it. Two mediocore franchises,Dallas and Detroit, volunteered to play on the Holiday and the short practice week. After a few years it turned into a traditional high profile game.
Other teams decide it would be in their interest to try to get on the tradition. The NFL has up to this point has told the other teams, you did not want to do this at the start and Dallas and Detroit stood up and volunteered to benefit the NFL therefore they get to keep the games.
OBTW Go Cowboys