Why did both korea and japan’s attitudes toward contact with the outside world change

The isolationist postures of these countries changed since they were forced to, at least economically and in fear the western civilizations would invade them otherwise.

At the start of the Meiji Era, the newly appointed government of Japan decided to open trade to western civilizations, in hope that it would prevent a future dispute and improve the country technologically and economically - all of this under the impression of the "black ships'" appearance, on which the foreigners first arrived on.

Regarding Korea, the United States gained a lot of influence over the southern division of the country (Now South Korea) after the Korean War. This regime caused the country to shape and develop in the likes of the U.S. until the 1990's, efficiently ending the isolated government previosly imposed by the japanese. Today, the northern division of Korea (Now North Korea) continues to be isolated, following the communist ideologies of their former influencers and allies, the russian communists.