North Korea was doing better than South until 60s, and not too bad even until late 80s. (They even sent aid packages to South Korea once in the 80s, when we were hit by a big typhoon. Largely as a propaganda show, but still.)
It then fell off a cliff.
I guess there were no longer any USSR to supply them with fertilizer and other stuff for free, and China retracted some of their support too. And they relied on centralized mechanized agriculture. Which in this closed off society led to a large scale famine.