I think the Soviet defense in the Far East always relied on geography more than force. Siberia and the Soviet/Russian Pacific maritimes are massive and massively underdeveloped - a paved highway was only
completed in 2015.
Beyond that, until the past decade or so, the Chinese military was decidedly backwards. A T-62 tank would be practically as effective against Chinese armor as a T-80, and a BMP-1 was still far more advanced as an infantry fighting vehicle than the more numerous, more primitive Chinese Type 63 APC.
Effectively, it was a secondary theater because it never needed to be more. Those Siberian forests would swallow entire Chinese (or American) armies.