I think it was a matter of they made him a offer(Carmine) and he probably had no choice.

Gambino wanted him gone and so did everyone in the life.... he set back the life justas much as Gotti did .

And Colombo had plenty on Gambino all that was terrible for everyone down to the associates ...not in the beginning but after Colombo was acting like he really was not a boss of a crime family saying all that shit.

Think about it who had the most to lose if he kept talking and who stood to gain with him quiet the mob !