Why does Northeast China develop late and its population density is low?

In the late Ming and early Qing dynasties, the southeastern part of Liaoning was a battlefield with a small population. After the Qing dynasty entered the customs, it called the northeast the land of Longxing, and it was forbidden to immigrate from the customs to the northeast. After the demise of the Manchu Dynasty, a large number of refugees from Shandong and Hebei flocked to the east. After the September 18th Incident, a large number of Northeast refugees fled to Shanhaiguan, further reducing the population of Northeast China. After the recovery, the Northeast experienced another three years of civil war. The above reasons have caused the late development of the vast and sparsely populated Northeast.