ROK had three main advantages that allowed it to be more prosperous than DPRK:
- It wasn't mostly destroyed by the US during the war.
- It has plenty of plains suitable for farming while DPRK has steep mountainous terrain.
- It wasn't sanctioned or embargoed.
Other events like the fall of the USSR and a major flood have also set DPRK back.
It will be interesting to watch if DPRK becomes the more prosperous nation long-term due to socialism simply being a better economic system and what this will mean for future reunification. But I am getting ahead of myself.