Lombardozzi was such a huge earner, and I guess that their suspicions were not "proof Positive" of his cooperation, that Carlo Gambino wouldn't sign off on it.

He was probably kicking up so much money to Gambino, that the old man didn't wanna lose the cash cow that Lombardozzi, "The Doctor" was.

Thats the only thing I can think of.

But who really knows? These types of things was largely mysteries.