A bunch of readers think the obvious answer is that he was simply stupid. I don’t buy it. First, he apparently had two college degrees and worked in the financial sector. Lord knows that doesn’t mean he had to be a genius. But a complete moron? I kind of doubt it. But even if he was a complete moron, So what? That’s what training is supposed to compensate for, right? Was he really so much dumber than the illiterate brainwashed shmucks so often used in suicide bombings throughout the Middle East and Afghanistan? I certainly find it harder to believe he was a lot dumber than the Mumbai terrorists, one of whom is interviewed on camera in the HBO special on the attack. Of course, many of those guys are just given the bombs pre-made. But still, Shahzad was in no rush. He could have at least bought the right kind of fertilizer. You don’t need to be a genius to be told to open the valves on the propane tank. Again, I’m not saying I know the answer. I simply think there’s a disconnect between the sophistication of the terrorist network that allegedly dispatched him and the lack of sophistication on display in the actual attack.
Update: Another round of explanations focus on the possibility he chickened out or grew conscience or some such. That’s certainly plausible. But the fact that he bought the wrong kind of fertilizer means that it had to be percolating subconsciously for a while. Otherwise, why go through with it at all?