darn, i should really go back and re-read the whole arc again, but...
the way i see it, fisk is in jail, he's been there for some time and lots of people are trying to kill him even when in jail. he was gonna get immunity from FBI for giving up matt, seemed like the right time then.
he's known about matts identity for years (i first discovered this in the parts of a hole arc as only started reading DD then), so the bigger question is why not reveal it before hand? fisk was ontop of the world, had an empire and then DD brought him crashing down to earth, had him incarcerated - fisk thinks, its time to use the get out of jail free 'daredevil's identity' card - cool plotting from what i can see