but in my line of work, the simplest explanation is usually the right one. criminals are stupid and are unable to keep their mouths shut.
they eventually seek help from people who are not as interested in criminal activity as they are.
informants are everywhere. i've arrested people for tiny little charges and they are ready to give up their mother to get out of a night in jail.
it is very difficult to plan/carry out a terror plot in todays NYC for these and many other reasons.
But these are not criminals, they are enemy combatants, spies, etc. Those tend not to be quite as dumb as ordinary criminals. Worse they are religiously motivated. (Although that does make them likely to talk to an Iman who might have a different view of the "Will of Allah").