ROK had three main advantages that allowed it to be more prosperous than DPRK:

1. It wasn't mostly destroyed by the US during the war.
2. It has plenty of plains suitable for farming while DPRK has steep mountainous terrain.
3. 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.