If they were released any earlier, most of the individuals involved would still be around. It doesn’t paint a pretty picture of the CIA and FBI, it kind of makes them look like a bunch of fools. Utilizing the mafia wouldn’t look good to a public that believes its government sticks to the law.
IIRC, the criminal underworld was enlisted in rooting out foreign agents in WWII. Nothing new about dealing with shady characters in the name of National Security.