Some simple observations:

Where are the most major centers of production/industry?
Where lie most major civic centers?
What are (traditionally) the major centers of political significance?
Where does the vast majority of party politicians; as well as dissidents come from?
Where does the CCCP draw most of its supporters from?
What regions are most reliant on food imports and hence are more likely to riot when food supplies fail?
What regions provide the bulk of armed forces as well as the bulk of armory stock?

The dependence of the USSR on its European territories goes so far at the time; that they cannot really rely on their Asian possessions to supply them -- even most major mining centers do no lie east of the Urals. Past the Urals is a country in which industry is still very much in its infancy.

So to sum up: what territories are most highly prioritized when it comes to exerting control over the populace and defending from [foreign] assault? I would not be surprised if you could find me a stat showing 95% of all Russian forces were congregated between Yekaterinburg and Warsaw at the time: such a deployment makes sense out of its own accord and does so even without a grand invasion plan.