I think there were a series of bad choices. The first was Don Carlo Gambino, he thought, to lead the family, need a Sicilian, chose Castellano, for this reason, and because was also his relative, instead of best Aniello DellaCroce, but it was a Neapolitan. On the death of Gambino, Castellano became boss, however, had not the qualities to be a great boss, he was a white collar that a streetman, it's attracted the hate of his own soldiers.
A typical case was DeMeo.Roy Roy was a butcher, and a serial killer, but he make a lot of money, is said he controll with the family DeCavalcante the 90% of pornography in New york.Castellano hated Demeo, but accepted the percentage on the drug (if in pubblic, he banned it), the carthefts and murders committed by his gang. But when Roy succeeded in concluding an alliance with the Westies, the the terrible Irish mafia gang, Castellano was forced to made him in 1978.
When they became uncontrollable ordered his death in 1983, and it is said that at the beginning, there was no one who had the courage to do kill him.Until DellaCroce was alive, Gotti didn't have the courage to do nothing, but to his death, there was Gotti still nothing from taking power.

Castellano's Death Certificate





Castellano's death scene photo