I'm surprised Cohen wasn't killed when Siegel was. They had to have known that Cohen would be mad about it, and that he would never be willing to work with Dragna. I'm guessing that despite Cohen becoming a problem, he was so much more business savvy than Dragna that they still wanted to keep him alive.

"...the successful annihilation of organized crime's subculture in America would rock the 'legitimate' world's foundation, which would ultimately force fundamental social changes and redistributions of wealth and power in this country. Meyer Lansky's dream was to bond the two worlds together so that one could not survive without the other." - Dan E. Moldea