To your question as to why Iraq over North Korea, it all comes down to the details.
Firstly, any military action would most likely result in huge civillian casualties. Both the North and South have enough conventional artillary pieces pointed at each other to more or less completely level the other nation in a matter of minutes. I do mean level. That includes capitals, major cities, even small farms. That’s before we start tralking about missile sites, or aircraft. Over the last 50 years of cease-fire, both sides have built up to ridiculous levels. I obviously don’t have access to estimated casualty reports, but a couple of posters have actually served in Korea, so they might be abvle to enlighten further.
Secondly, North Korea is still allied with China. While China certainly doesn’t want to see North Korea destroy it’s gains with the West, made over the last few decades, neither do they want to abandon it. They still, to my knowlege, have pacts and treaties to defend each other. While China’s military is woefully underequipped, they outnumber ours by a huge factor. So, we have the possibility of intervention by China, which turns an attack on Korea into a huge shooting war.
Thirdly, and this one is pretty important, N. Korea already had nukes, and claimed to have delivery sstems that could reach not just the South, but all the way to Japan, and further. That means that, as a last resort, Kim Jong Il turns Tokyo into glass. Obviously, something we’d want to avoid.
Lastly, it comes down to the availability of other options. With both North Korea and Iraq, our relations were strained at best. However, there were strong regional forces that could assist us with talking to North Korea. China, Japan, and South Korea all pressed for seats in talking the DPRK down from the ledge it crawled out on. Previous attempts at talking with Saddam had produced a stack of UN Resolutions, with little to no real results. We hadn’t tried much talking with N. Korea.
I don’t think you can point at just one reason as to why we attacked Iraq and talked with N. Korea. There are a good many reasons to do so. While it’s not good for the people of the DPRK, there’s unfortunately little we can do militarily that wouldn’t involve millions of people dying on both sides. With Iraq, the death toll would be in the thousands.