I think the NSA could crack this phone with a little work, but chooses not to do so simply so the FBI can expand its power.
Above all, the problem here is caused by a failure to secure the borders against Islamist infiltration, and to clean out Muslims who are tied to terror. And I am sick to death of having both my dignity and liberties insulted and constricted because the government prefers to treat Americans as criminals and foreigners as privileged.
In fairness to the NSA, they were not asked for assistance, because if they'd cracked into the phone the FBI wouldn't be able to use it in an attempt to set a powergrabbing precedent.