Except... they kind of did. Prohibition created revenue opportunity for gangs and organized crime. Ending prohibition removed this revenue, replacing the speakeasy with legalized and regulated bars and liquor stores, effectively cutting off this revenue stream.

Sure the Mob didn't magically disappear overnight, but it began a decline they never recovered from. Replacing that revenue stream isn't as easy as "just" finding another racket, there is a lot of difficulty hidden in that "just".