Law enforcement (FBI, local and state) probably had plenty of charges that they could have leveled against Pentangeli. But, as a caporegime, he probably had insulated himself in the same way Vito did--giving orders to one person at a time so that law enforcement could not produce a corroborating witness against him. I'm guessing that Cicci's defection provided law enforcement with that corroborating witness. And, Michael's "betrayal" gave Frankie further impetus to cooperate. Never underestimate "revenge" as a motivation in the Mafia.