Castellano was both Gambino's cousin and in-law. That probably is the primary explanation.
Perhaps Gambino justified it to himself by reasoning that the future of the Family would have been better secured by the relatively white-collar/smooth Castellano instead of the frightening and rough around the edges Dellacroce.
There's a lot of examples in corporations where the number 2 guy feels entitled to the top spot because of his deep loyalty and long experience but precisely because he is so wedded to the old way of doing things, doesn't get the nod.
"When the snows fall and the white winds blow, the lone wolf dies but the pack survives." Winter is Coming Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die. As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.
