In perry anderson, he mainly explains it via peasant frontier: where state authority was established runaway peasants would likely be caught, so there was diffuse movement of peasants from formed state to the west (cossacks) and east. But this depends on which time period you talking about, because initial state involved eastern expansion (in 1500) was driven by one or the other khanate raids resulting in wars
Siberia was diffuse at first (connected with fur trade as mentioned and lumber maybe), and then centralized to reach pacific, but that was much later
Invidious, bby, cmon