If I'm correct, Casso never wanted to be the boss himself, and felt most comfortable in his role as consigliere. He wanted to stay under the radar for de feds, but in reality he was the one who pulled the strings. If this was true, I don't see why he wanted to become official boss. And besides, no rat becomes a boss or stays a boss for long.
"It was between the brothers Kay -- I had nothing to do with it."
