According to one FBI informant(likely Lombardozzi), Castellano was made acting boss in 1967 at the same time Dellacroce was made underboss.

So in a way (if this report is correct), Castellano was "over" Dellacroce for sometime prior to Gambino's death(though I realize Dellacroce had lots of independence), and in the eyes of many, Castellano would have been seen to be the natural successor to Gambino. (I'm not saying Castellano was a better choice than Dellacroce).

I'm guessing too that Gambino figured that Castellano as boss would better protect the rackets of Gambino's sons.