At first it was because of harsh economic conditions in northern China and allowing the settlement of some land in Manchuria was seen as a way of relieving hardship by increasing the amount of land available to be farmed.

But in the mid-late 19th century it was largely because of the Russian Empire, who desired and coveted territory in Manchruia and to whom the Qing was forced to cede major parts to in 1858 and 1860. The Qing government realized that the area was very thinly populated and therefore should Russia make a grab for even more territory there wasn't that much Qing could do to contest it. However, if the area was settled with Chinese immigrants than the Qing would have a much much better claim to the area and conquest by Russia much harder/less appealing. Hence lifting of restrictions on immigration and by the 1900s 10s of millions of Han Chinese have settled in Manchuria.

Because the Russians/Russian Empire