The reason why 1 is because of Feng Shui. At that time, when Nurhachi established the post-Jin regime, in fact, the whole Manchu people regarded Changbai Mountain as the birthplace of Manchu culture. With the gradual expansion of Nurhachi's influence in the Northeast, it was not until later that he led the army into the Central Plains. Therefore, after the establishment of the Qing Dynasty, the Qing people regarded Changbai Mountain as a place to revitalize the Manchu. According to some geomantic theories, the mountains in Changbai Mountain are similar to dragons, so some people regard the mountains in Changbai Mountain as Long Mai.
The second reason is the unique geographical location. At that time, the Qing dynasty not only banned Changbai Mountain, but also banned most of Northeast China. The reason for this is that the distance between Northeast China and Outer Mongolia and North Korea was very close at that time, and the location of Northeast China was also crucial. It has a very obvious strategic position. At that time, the Qing dynasty was afraid that the Han people would collude with the Mongols and Koreans, so it wanted to build the Northeast into a large military base. Only by blocking most of the northeast region can we isolate the opportunities for Han people to communicate with Mongolia and North Korea. At that time, Changbai Mountain was very rich in products, as well as animal and plant resources, the most famous of which was ginseng in Changbai Mountain. Changbai Mountain was forbidden to open to the outside world by the royal family of the Qing Dynasty, mainly because the royal family wanted to turn the northeast into its own treasury.
The third reason is political factors. At that time, the rulers of the Qing Dynasty wanted to build the Northeast into a place where they could save their lives at any time. Once the troops of other countries invaded the capital at that time, they could lead the royal family to retreat to the northeast and continue to rule.