There's no such a thing as a humanitarian intervention. As sibling posts say, there's plenty of market for it, if a country really wants to do it.

Even ignoring this aspect, there are two reasons for not waging a war to NK:

  1. NK is under China's protection, primarily for acting as buffer country (this is a valid political reason)
  2. NK does have offensive power; even without nuclear warheads, due to geographical positioning, it can do a huge amount of damange to SK.