To put it simply, before the Japanese got their hands on the Northeast, the Ming and Qing Dynasties had been operating in Jilin City for hundreds of years, and Jilin City was already the center of Jilin Province and even the Northeast. But during World War II, the Japanese chose Changchun as the central city of development. After the founding of New China, Changchun was the central city.
However, after the Japanese moved in, when choosing a new central city, they deliberately avoided Jilin, an old center with deep influence in China, and instead built a "new Beijing" on Changchun that was really built and controlled by the Japanese from scratch. With the introduction of a large number of Japanese funds and resources, Changchun has risen rapidly, not only leaving Jilin, the old provincial capital, but even becoming more advanced than big cities in Japan.
After the retrocession, Chiang Kai-shek, as a man with strong national feelings, regained his identity because the "new Beijing" built by the Japanese was a "national shame" and the old provincial capital Jilin was reopened.
A few years later, after the founding of People's Republic of China (PRC), the decision-making starting point of the new China government was more "pragmatic". Since Changchun has been built into a very developed big city, there is no reason to make an old city with a history of hundreds of years as the provincial capital. So in the 1950s, the provincial capital moved to Changchun, and Jilin became an ordinary prefecture-level city.