It's true that Hoover played the ponies, but that's not why he didn't go after the Mafia to any large extent. He didn't go after the Mafia because he thought the Mafia was limited to local jurisdictions and were not interconnected, and therefore were not federal crimes. In the 1940s the FBI did start an investigation into the death of Bugsy Siegel and the Chicago Outfit, but it was ordered shut down by the Attorney General. There were investigations of individual mobsters, but these were mostly newspaper article clipping operations. All that changed after the 1957 Apalachin meeting. That made him rethink his position and realize he was mistaken. No one forced Hoover to investigate then except for Hoover. That's when they started installing bugs and developing informants.