It's not a question of whether either has the military power to overrun DPRK. Both could in a matter of hours a few days. A lot of it is economic. What happ

It's not a question of whether either has the military power to overrun DPRK. Both could in a matter of hours a few days. A lot of it is economic. What happens to the ~24 million DPRK citizens who are very low skilled? Whatever country takes them on is taking on a gigantic uncertainty and costs for literally millions of people. During famines, North Koreans have literally gone to the hillsides to get grasses to eat. Many are malnourished, roadways are poor outside of Pyongyang because they need travel permits to travel within country (so the roads are not used much). To make anything of the country would require huge infrastructure costs. Many of the roadways are also suspected to be rigged with explosives so if/when a land invasion occurs, US and South Korean troops are slowed down significantly. China doesn't want to do it because the buffer between the "US sock puppet" called South Korea is nice to have. They have had trade going with North Korea for years, and recently have gotten annoyed by the rhetoric and BS spewing from their buffer zone. With Seoul very near the border, South Korea stands to lose many billions of dollars in assets from artillery once the invasion starts.