I believe the decline came in the 70s later you still had some powerful Russian and Israeli groups in the US.

Several notable Jewish-American mobsters provided financial support for Israel through donations to Jewish organizations since the country's creation in 1948. Jewish-American gangsters used Israel's Law of Return to flee criminal charges or face deportation. Notables include Joseph "Doc" Stacher, who built up Las Vegas by pairing the Jewish and Italian Mafia into a national organized crime syndicate. Prime Minister Golda Meir set out to reverse this trend in 1970, when she denied entrance to Meyer Lansky