Siberia was conquered by Russian-Turkic cossack gangs for a singular, purely economic reason: furs.
For Russia, the fur exports in the 17th century played about the same role as petroleum now. By the start of the 18th century, they gave requisite cash flow for Peter the Great’s wars against Swedes and Crimean Tatars that resulted in the rise of the Russian Empire.
For the Chinese, furs had no particular value compared to other colonial wares. There was little else that could motivate them to spend precious resources on controlling these vast, barren expanses to the north. This is why they preferred to let locals harvest the territories for pelts without imposing military control or a dedicated arrangements for taxation
Besides, the Chinese didn’t possess the military technology for penetrating these territories. Cossacks, on the contrary, were made for the task. They operated as small, autonomous, highly mobile troops with firearms. They knew how to quickly construct ad hoc river vessels for traveling longer distances on the water (thank you, Varangians!). They combined this with traversing the wilderness between watercourses on foot—the task seemingly impossible for the massive, cavalry-based Manchu troops.
When Cossacks imposed control over Siberia and the northern part of the Far East, they took over much of the deliveries to China. For them, this was a high-volume and mostly low-margin business that included large deliveries of squirrels and hares. Mink, sable, bever, sea otters were typically dispatched to Muskovy, where they fetched a better price. Later, when permanent logistics were established along the Pacific coasts of northern Asia and Alaska, they added sea otters to this lineup.