The proximity of populated areas of South Korea, and the touchiness of China over intervention in the area, are particular problems. If it weren't for those, the U.S. would probably have bombed the North at one point or another, especially once the nuclear tests gave a plausible excuse to do so. If anything there was a better argument for attacking North Korea than for its fellow "axis of evil" member Iraq (on either humanitarian or "WMD" grounds), but Iraq was a much easier target. It wasn't really for any lack of interest on the U.S.'s part: for a few years, very pro-intervention neoconservatives had a strong role in the Bush administration's defense policy, and they investigated North Korea options, but presumably couldn't arrive at one.
