Nick says that "this has been a story of the west , after all" What do you think he means by that?

Nick is contrasting the decadence of the east with the higher moral values of the west. Gatsby and Nick were both midwestern boys, who head east for completely different reason. The east corrupted and destroyed Gatsby, and because of this Nick feels the story has been all about the west. He feels the need to return to reality and home.