From Underboss

"If he’d had to bet on who might
turn, it would have been the most marginal defendant, Frank Locascio. But Locascio had a serious problem. His son was a made member of the family. If Frank flipped, he would be signing his son’s death warrant."

D'Arco flipped before Gravano and his son was a made member. IDK why this would stop Frankie.