Same reason that the championship in baseball is called the World Series. They're the best in the world at American football.
I do like rugby, but I love football more. The strategy is unparalleled in any other game, (Rugby and soccer have strategy, but nothing like the coordination of play-by-play action) and it strikes the perfect balance between suspense and speed. There's absolutely nothing like watching a fourth-quarter drive down the field in the last two minutes.
(Pats fan. Good game, Giants. It was an awesome game. Good, hard, clean football)