I think there were less arrests because they compromised the SFPD. The FBI complained for decades about the SFPD frustrating their investigations of Lanza and his crew. Even in the 1990s the FBI went on record saying as much during the big cocaine busts that went down at that time, when they busted Balistrieri and sons and Sergio Maranghi. They weren't able to get to the main target, G. Toracca, because elements of the SFPD protected him.

The history of the San Francisco Police Department is intertwined with the history of organized crime in S.F. from the very beginning. The Atherton Report of 1937 sheds some light on it.