Kim Il Sung wasn't the first choice as the future ruler of North Korea. The first northern provisional government, the Administrative Committee of Five Provinces, was chaired by Cho Man-sik. Cho was popular, and broadly supported by the people in northern Korea. He had organised the pre-Soviet provisional government in the north, the Provisional People's Committee for the Five Provinces, and as the existing provisional leader when the Soviet occupation forces took over, became the provisional leader under the Soviet-supported provisional government.

Cho was not a communist, and distrusted the Soviets, so their working relationship was somewhat uneasy. if he had remained as the leader of North Korea, he might not have survived (politically survived, even if he was still alive) the probable communist takeover of North Korea. As it was, an issue where he would not compromise quickly arose: the Moscow Conference of December 1945 had decided that Korea was not ready for independence, and there would be a five-year trusteeship period, with Soviet and US administration of the north and south respectively, before holding elections for a unified independent Korean government. This decision by the Western Allies and the Soviets was imposed on the Koreans, and was seen by the Koreans as thwarting their desire for immediate self-government and unity. Cho strongly opposed it. He was arrested in January 1946, and his provisional government was replaced in February by the Provisional People's Committee of North Korea, chaired by Kim Il Sung.

There were other communist potential candidates for leadership, but Kim Il Sung was (a) the best known and most popular, due to his relatively high-profile anti-Japanese fighting, and (b) he was willing to go along with the trusteeship plan.

A similar thing happened in South Korea, with Syngman Rhee being the top candidate for leadership who was prepared to work with the trusteeship plan. As it turned out, both South and North Korea became independent self-governing states well before the intended five year trusteeship period finished, and Korea remained divided. While both Kim Il-sung and Syngman Rhee were willing to go along with the trusteeship plan, and the (supposedly temporary) division of Korea, both planned for Korean reunification by force ASAP.

For more on this, see my older answer in https://www.reddit.com/r/AskHistorians/comments/d65wxh/how_were_the_governments_of_north_and_south_korea/