The whole idea of the show was that Tony (as the main character) was a powerfull mobster and a leader. Leadership and responsibility that comes with it were major themes in the series, if Tony was only a captain in one of the smallest families it wouldn't be so interesting. It would be different if he was let's say a captain of a powerfull Genovese crew in NJ, but Chase wanted the series to represent the DeCavalcante NJ family. Anything lower that the boss would make him look like some small time mobster IMO.