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.
On one hand, you don't really see law enforcement officials making those rosy predictions about destroying the mob in another 5 years like they did back in the 1980's and 1990's. At least in New York, they appear to have realized this is going to take a lot longer than some expected.
On the other hand, they've had to reevaluate their priorities over the past decade. Not just in regards to terrorism in the wake of 9/11; but also in regards to transnational organized crime, which has become the Justice Department's #1 priority in that area.
From 1986 to 2010, convictions involving La Cosa Nostra was 78%, while convictions involving emerging organized crime groups was 22%. However, since 9/11, from 2002 to 2010, convictions involving La Cosa Nostra has been 60%, while convictions involving emerging organized crime groups has been 40%.
In short, it seems like the "status quo" the feds have settled with, at least for now, is to simply use the minimal (some would argue too minimal) manpower needed to keep the mob in check. Keep the indictments coming, keep the mob from regaining a lot of influence, while waiting for attrition to take it's toll.
