They rebuilt a lot of that in the decade after the war with extensive aid from Russia and China. Their economy was actually doing better than South Korea's up until maybe the '70s or even '80s (it depends on what metrics you are looking at).
Where it started going seriously off the rails was in the '80s when they adopted a policy of radical self-sufficiency. Unfortunately they can't really be self-sufficient, at least at their current population levels, because the climate and geography limit the amount of arable land and limit how much can be grown on it. They get winds from Siberia bringing bitter cold and heavy snow, making it so they can usually only get one crop per year (compare to two crops per year which is possible in much of South Korea).
So they remained heavily dependent on Russia and China. When the Soviet Union broke up they lost most of their Russian aid, and that really hurt. They never really recovered from that, and their infrastructure suffered as part of the general poor economic conditions.
But that still makes them 20 years behind the times. And then:
"Russian accusations of indiscriminate attacks on civilian targets did not register with the Americans at all. But for the North Koreans, living in fear of B-29 attacks for nearly three years, including the possibility of atomic bombs, the American air war left a deep and lasting impression. The DPRK government never forgot the lesson of North Korea's vulnerability to American air attack, and for half a century after the Armistice continued to strengthen anti-aircraft defenses, build underground installations, and eventually develop nuclear weapons to ensure that North Korea would not find itself in such a position again. ... The war against the United States, more than any other single factor, gave North Koreans a collective sense of anxiety and fear of outside threats that would continue long after the war's end"
Where it started going seriously off the rails was in the '80s when they adopted a policy of radical self-sufficiency. Unfortunately they can't really be self-sufficient, at least at their current population levels, because the climate and geography limit the amount of arable land and limit how much can be grown on it. They get winds from Siberia bringing bitter cold and heavy snow, making it so they can usually only get one crop per year (compare to two crops per year which is possible in much of South Korea).
So they remained heavily dependent on Russia and China. When the Soviet Union broke up they lost most of their Russian aid, and that really hurt. They never really recovered from that, and their infrastructure suffered as part of the general poor economic conditions.