In the spirit of speculating based on known movie characters, here's a theory for you: Michael had Don Altobello manage things in NYC until he stepped aside for Joey Zasa.
That's an interesting one, Woltz. He came in out of nowhere in III. He was Connie's godfather, which means he was close to Vito. He also was aligned with Zasa in his betrayal of Michael. Anyway, it's the best answer to a question that has no provable answer.
Ntra la porta tua lu sangu � sparsu, E nun me mporta si ce muoru accisu... E s'iddu muoru e vaju mparadisu Si nun ce truovo a ttia, mancu ce trasu.
