Those gangs fell apart for the same reason the mafia is crumbling today. There were more opportunities besides crime for would-be Irish and Jewish hoodlums. On top of that, you had less Irish and Jewish immigration to fill the ranks of these organizations and the old neighborhoods. All of the shitty neighborhoods where street crime would thrive are gone (Hell's Kitchen, LES, etc). Well, there's still places like Harlem, Brownsville, and East New York. But even Harlem is gentrifying.
Better opportunities and assimilation plays a part in gangs falling apart.
One difference with the Italian gangs is the life-long oath. After prohibition, many of the non-Italian bootleggers like Joseph Kennedy and Costello pal Bill Dwyer went legit. They went legit because they could. The Italian gangsters were stuck. What would happen to Frank Costello if he announced to everyone that he made enough money and wanted out?
