If the US were to take action against N Korea to rid it of nukes and a harsh dictator you would have to ask the question of why no interference in Zimbabwe and other nations where the citizens have suffered terribly over the years and have been crying for help.
There are a lot of reasons we ignore Africa, one is Mandela and SA. We allowed him to become a hero and he set fourth some policies of non-external intervention that prevents a lot of things from happening. The entire African Union needs to do something about the problem, not the US. The AU has had UN backing and both of them could make a difference but as George Clooney found out the hard way, the UN doesn't want to end the bloodshed there.
As for North Korea, in comparison to say Iran, the NK people have a lot less freedoms and a lot less chances to change their government than the Iranians. The Iranians have loosened up a bit because of the Iraq/Iran war but NK hasn't been at war with anyone else for 20 years when Iran changed governments.
China warned Kim and so did Russia, neither of them need a war on their borders at this moment. The world would surely crash and burn and then both of them would lose millions in hard assets. The UN won't do a thing and again we need not to return to the Albright appeasement plan but to the Bush multilateral plan. It worked and worked well.
But again we have the Messiah, the annoted one who will solve all of our problems and hand them to our great grandchildren - wasn't it the mantra of the hippy generation "live for today, don't worry about tomorrow"?