Casso says to fed that apart eppolito and caracappa he know a rouge fbi agent and in change of the proofs he want to go in the witsec. So the fed don't believe him about the agent but even if the FBI would make a deal, Casso was unpresentable the g men Call him Lucifer and will be his word against a fbi agent word.
For many people fbi cover-up the story after the devecchio scarpa affair or connolly bulger story. Who know ? Maybe the feds thinked that already had d arco and casso was useless and danger.