North Korea did drop reunification as their goal from their constitution.

Its possible North Korea has accepted that reunification will never happen any time soon, and would rather keep control of the swath of land and captive peasants they currently hold.

Additionally, South Korea is facing an apocalyptic fertility crisis that will already end the country if they don't effectively open up to immigrants and make life easier for the average Korean. Since their country is infected with terminal capitalism, we probably won't see that, but ending the war reduces stress.