What I think is that they have established a status quo. The feds know that they can never completely annihilate organized crime. If they destroy the New York Mafia other groups will simply fill the vaccuum. Better to have an established organization that they know to be in charge to some degree and maintain order within the underworld than lesser known foreign groups.

That big bust in early 2011 was just for the show in order to justify to the public why they would reduce the number of FBI squads dedicated to fight the Five Families. And the Mafia probably realises not to become too opportunistic and exploit the situation, because it can change back to what it was within a heart beat.