Or how about why Sonny Black was killed, and not Lefty, when the Bonanno's found out about Pistone?
The movie "Donnie Brasco" wrongly overemphasizes Brasco's "warm" relationship with Lefty. In the book, Brasco showed little respect for Lefty, whom he regarded as a whiner and parasite. Brasco expressed near-affection for Dominic (Sonny Black) Napolitano, who treated him like a kid brother, let him live in his apartment, and apparently gave him the assignment to kill Bruno Indelicato that would have led to Brasco's being made. Therefore, Sonny Black was at least as culpable as Lefty for Brasco's "betrayals," and probably more so.
But, sez I, rules, schumules. As in everything else in the Mafia, "rules" take a backseat to money and power. After Brasco was exposed, Bonanno boss Philip (Rusty) Rastelli had his ass in a sling with the other Mob bosses. How was he going to prove to the others that the Bonannos were serious about exacting vengeance for this incredible gaffe--by whacking a
cocozz' nobody like Lefty? Or an underboss, like Sonny? Remember, too, that Sonny Black made a big power move by whacking his rival, Alphones (Sonny Red) Indelicato and his cohorts. He also raised his profile tremendously--potentially bad stuff in the Mob, where envy, fear and greed are the handmaidens of everyday life. Probably all the other Bonanno capos and hierarchy went along with making Sonny Black take the fall for Brasco because they'd all benefit: Some of them (like, ahem, Joey Massino) would move up. Others would breathe easier with Sonny sleeping with the fishes, knowing that he couldn't rat them out to law enforcement to save his life.
Yes, Lefty could have been whacked in prison. But, after the Brasco affair, the Bonannos were in disarray. Arranging a murder in prison costs money and effort. And anyway, Lefty kept his mouth shut--even on his deathbed.