What did you think of the Double Cross book/hypothesis?
Actually, that is one of several books that lead to my conclusion. That and the fact the FBI has wiretaps of Mafia heads, plotting revenge against Joe Kennedy where one of them remarks, “he'd old, you want to hurt him, take away the thing he's always wanted.”
Being President, and then later having one of his kids be President, was his life long dream. The guys that ran the Mafia back then lived by the feud. You cross them, they will get revenge.