I think the fact that the American organization is the only criminal organization that shares the Cosa Nostra/Mafia name with the Sicilian one says it all. The Sicilians definitely formed the most powerful faction and the Neapolitans, Calabrese, Apulians and others had to follow suit. Another giveaway is that American families like Gambino and Bonanno have had their specific "zip" factions throughout the years, which indicated strong Sicilian ties. Such thing has not really been the case with the Neapolitans or Calabrese. It seems that in the Italian-American underworld Sicilians retained more of their specific cultural traits which set themselves apart as "Sicilian" while Italian immigrants from Campania, Calabria, Apulia, etc...tended to get Americanized quicker as "Italian American".

The Palermo and Trapani groups seem to have had the most pull in the American crime families. Those from Agrigento have more pull in Canada.