Why was the division of Korea originally intended to be temporary?

The division of Korea along the 38th parallel was originally intended to be a temporary administrative measure following the end of World War II.12

At the conclusion of the war, the United States and the Soviet Union agreed to use this line as a boundary to facilitate the surrender of Japanese forces: Soviet troops accepted the surrender of Japanese forces north of the line, while American forces did so to the south. This arrangement was designed to be a short-term logistical solution while the Allied powers made necessary preparations for Korean independence, as pledged in the Cairo Declaration.321

However, this temporary division became permanent due to the onset of the Cold War. Diplomatic efforts to establish a unified government, such as the Joint U.S.-U.S.S.R. Commission, failed to reach an agreement, leading to the formation of two separate, ideologically opposed states in 1948.124