The only explanation I can find for the lack of a robust reponse to North Korea (a country which poses a far greater risk to world and regional peace than, say, Iran) is that the South, the USA and their other allies are hoping North Korea will disintegrate on its own under a tidal wave of economic collapse (one more famine like the one in the mid-1990s will do it) and they can send in peacekeepers and humanitarian aid rather than mount a full-scale invasion.

Certainly, it seems likely that South Korea simply letting the North get away with sinking one of their warships earlier this year with no retaliation encouraged the North to take further action. A more robust South Korean response could dissuade further escalation. OTOH, it could also trigger it, considering that the North Koreans are even more jittery than normal. Certainly not a decision I'd like to make.