Early in the past millennium, Kiev was probably the largest, most modern city in the former Soviet Union, at least until the Mongols came. One might have expected it to be the capital of the Soviet Union, with its location on the Dnepr River, and relatively easy access to eastern and southern Europe, plus the Black Sea (via the Dnepr)? Or did Mongols change all that?
Another, logical "main city" is St. Petersburg (formerly the capital), with its coastal position, and window to western Europe.
Looking at the capitals of Europe, they are mainly located on the coast (Oslo, Stockholm, Helsinki) or along a major river (London on the Thames, Paris on the Seine).
Moscow, by comparison, seems to be in the middle of nowhere. The only comparison I can think of is Poland's Warsaw, which became the capital when Poland and (the former) Lithuania merged, with Warsaw being near the center of the combined country.
Or is that why it became Russia's capital, and largest city, with "equal access" to the Baltic and the Urals, the White and Black Seas?