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.