Why did the Qing Dynasty use Long Mai as an excuse to prohibit Han people from entering the Northeast?

The main purpose of closing Northeast China in Qing Dynasty was to maintain the living customs and riding and shooting traditions of the Eight Banners and build Northeast China into a strategic base to control the whole country. The Qing dynasty considered the northeast as its "land of prosperity" and they lacked confidence in long-term rule. In the future, Jiangshan can't sit still, and they can move back to the unspoiled base area. It is also to protect the local mermaid hunting life, so Han people are forbidden to live in Manchuria. However, the number of troops stationed in the Eight Banners of Manchuria is scarce, so it is a problem to patrol all the time. After the Qing dynasty's own statement entered the customs, Manchuria was almost uninhabited, so the other Eight Banners had to stay in Manchuria in turn. The policy of forbidding Northeast China in Qing Dynasty was a special policy of Manchu aristocrats ruling Northeast China, which became the core of the policy in Qing Dynasty. The policy of banning trade reflects the basic national policy of worshipping Manchu in Qing Dynasty, which undoubtedly hinders the development of Northeast China.