There's a pervasive belief in this society that perfection is possible. So if something bad occurs, it can never be cause we just got unlucky. It must be because something went wrong and someone is at fault, and then things must be fixed. Sometimes, though, this simply isn't true. Sometimes it's better not to fix things: either there is no fix, or the fix is more expensive than living with the problem, or the side effects of the fix are worse than the problem. And sometimes you can do everything right and have it still turn out wrong. Welcome to the real world.
If there is a lesson in the Christmas non-bombing, it is that you can't defend against every nut out there any more than you can prevent every highway smash-up or household accident. To live is to accept some modicum of risk. To live in an advaned, industrailized, technological, hegemonic, militaristic, imperial society in the twenty-first century is to accept with fair certainty that someone, somewhere, wants to blow you up, like, right now. This is, as they say, the cost of doing business, an inevitable outcome of the system in which we live, work, and pretend to thrive.