The people of North Korea are brainwashed into believing that there leader,Kim Jong-un is a God. Here are children crying at the opportunity to be around him.
To keep the people from rebelling, the government feeds the citizens lies with propaganda, like the picture here. Due to this most citizens have developed a hate towards the USA.
The reason for no extreme intervention by any other country is the mass weapon arsenal which includes nuclear bombs. The countries fear an assault to their own people, and would rather not take the risk.