Why did the annexation of Siberia and the Far East happen peacefully and relatively quickly?

The main reason is that there were no strong states on this territory capable of providing strong resistance to the Muscovy and the Russian Empire in the 16th-18th centuries. For example, the Siberian Khanate was defeated in 1582 by Ataman Yermak with a detachment of 540 Cossacks. After his death in 1585, detachments of 200-300 archers came there. The territory of Siberia and the Far East was sparsely populated, and the population was mainly engaged in hunting, fishing and gathering, therefore, they were simply forced to pay tribute, and these tribes did not particularly interfere with the migrant farmers.