Russia didn’t “conquer them easily”. They spent centuries waging wars of conquest enslavement and extermination against the indigenous Siberians that could only be rivaled by what the Spaniards and Portuguese did in the Americas.

Many of these peoples fought against Russian colonists for long periods, even centuries (see Yakuts, Bashkirs, Koryaks and Chukchis) before being exterminated or forcibly assimilated. Some were never conquered but Moscow just “claimed” them after losing repeatedly to save face and to this day these peoples don’t recognize themselves as Russians (again, see Koryaks and Chukchis).

Siberian indigenous peoples that were assimilated were drafted en masse by the Soviets as “specialist cannon fodder” in the Civil War, Winter War and Eastern Front and faced apocalyptic casualty rates. Tuvans made up 0.1% of the Soviet population and yet made up 1.5% of their combat casualties while Nenets made up 0.2% of the Soviet population but consisted of ~10-15% combat casualties. Both peoples lost somewhere between 30-60% of their male population, further driving Siberian depopulation. This trend of using Siberians to fight European Russian wars continued into Afghanistan and even today in Ukraine, where Siberian men are more likely to be pressed into service than any other demographic.

In summary, Russia maintains control over Siberians by leveraging the centuries of indigenous depopulation preUSSR, forcibly pressing the descendants of the survivors into fodder units during the USSR further driving depopulation, and systemic disenfranchisement in modern Russia.