The Soviet Leadership, either worried as a result of the ease in which the Nazi's invaded the Westerns SSRs or feeling more threatened by the Allies/NATO right after the war (quicker and/or more nasty parting of ways), decides that keeping the capital in the West is simply to dangerous, and perhaps it also dawns on them that the seat of power and authority being in one of the most populated parts of the Union could be dangerous in the case of future revolt, thus making them decide the best idea is to relocate the capital East, most likely to a large, purpose built city.
If Japan falls to the Soviets and China's less of a problem their's a slight chance they might just relocate to Vladivostok, but given its position I figure even then they'd go with a purpose built city far away from any threats.