Yuri Semyonov's The Conquest of Siberia says the main motivation for the Russian empire's eastwards expansion was the acquisition of furs (in the form of yasak, the fur tax assessed on native male subjects). Geopolitics was a relatively minor concern; no other power was rapidly expanding towards Russian territory, and the imperial military was in any case often occupied with wars in Europe.
Of course, the empire also had a state religion: the Orthodox church, which midway through the era of expansion, suffered a schism. Did it provide doctrinal justification for the conquest? Could it afford to missionize the new territory? Did the creation of Siberian dioceses such as the bishopric of Irkutsk change church politics?