It was probably a commission order and lets imagine that Costello was possibly the only one who voted against it. Same as the "legend" regarding Lansky and Bugsy. If Chicago and the other families voted for Morettis removal, than that was it. No matter how powerful Costello was, still he had to follow protocol. New York was different than Chicago in term of territories, meaning the NY families operated in all areas, while Chicago was more territorial. This means that Moretti was able to give information not just regarding the Genoveses or Chicago, but also about other families too.