NK is a complete basket case, with a population that has been brainwashed by 70+ years of misrule, a development level probably below China in 1949, and they have a different language and culture.
Right now, China gets the benefit of a buffer state without the downside of fixing generations of problems. They would take the blame for every problem.
Did you know that the goal of reunification is explicitly written into South Korea's constitution? Yet I've spoken to younger South Koreans -- even they don't really want to reunify with the North, because it would significantly lower the reunified country's standard of living.
It took an entire generation for East Germany to reintegrate with the West, and even now GDP per capita is about 30% lower in the east. And East Germany was only about 20 years economically behind the West. For NK, multiply that problem 10x.
