Originally posted by: yukichigai
Two reasons.
One: attacking North Korea would create a power vacuum. Any history buffs know that this was one of the main causes of World War I. The assassination of Archduke Ferdinand (wasn't that his name?) created a "power vacuum"; the power weilded by the Archduke was holding several countries at bay from each other country. With the Archduke gone there was no block stopping a war from breaking out, and added to the internal struggle for power the whole thing snowballed into World War I
As it is North Korea is indeed a nutso whackjob country headed by an even more nutso whackjob leader. But it's also a buffer zone between several countries and a piece of land that will be hotly contested. True, the US could just step in and say, "it's ours, we attacked, we get it," but that would invoke the ire of more powerful nations. Leaving it to sort out its own government would also leave it vulnerable to covert coup attempts by foreign powers. And with the power of the country weakened the rest of the surrounding countries would see it as an opportunity to settle old/new scores.
Two: to put it bluntly, North Korea can fight back. Sure, it won't do much good 'cause we'll more than likely nuke 'em 'till they glow, but Kim Jong Il is just crazy enough to fire every warhead he has at a major US city before the sand turns to glass.