My guess is because he has thousands upon thousands of artillery pieces trained on our ally. There are way too many to take out quickly. They would cause the loss of hundreds of thousands of lives… maybe millions. That's unacceptable unless or until he does something truly crazy. This is purely my opinion though.
We can not attack him without making ourselves a target. Right now North Korea has the backing of Chine and if we attack North Korea China will step in to defend North Korea and that could start WWI. That is a very good reason to not attack North Korea unless they attack first. Then maybe China will not step in.
KN,
I don't think China is as eager to step in as in the 1950s. Then, we had just finished supporting the KMT in the civil war and that knowledge was fresh in China's mind. They did not want us at their border. I think they still don't… but if we said this is about regime change and not a unified Korea they might agree. That said, I don't think South Korea would agree to that. Especially knowing the devastation a war on the peninsula would bring to them.
KN,
I think they will be very generous in their definition of what an attack is. They know a war with the US would destroy them and they think they are going to advance ahead of us economically and possibly technologically soon… they don't want to put that in jeopardy for North Korea and they won't. There may be some bluster on China's part, but I think if North Korea pushes too far China will let us do what we need to do.
Well with the North Korea dictator telling the US to expect some unexpected special deliveries I think he has succeeded in making the threat level between us and the them more on edge. I would not be surprised to see this chicken head keep thinking he could win against us.