It was a combination of things. To begin with, I don't think that Accardo had as much power as people think he did, but at the same time I also think the same of Giancana.

At the time, Giancana was under a lot of pressure from the feds and had served some time for contempt of court. He was afraid that he was going to be constantly harassed to testify and then jailed because of contempt every time. I think it was more of a mutual decision between the three of them for Giancana to step down and take a leave of absence, but unfortunately for Sam, by the time he got back his power structure was gone and many of his key guys had been jailed or had died. Aiuppa and Cerone took it a step further and had him eliminated just in case he had aspirations of trying to reclaim the throne.