After World War 2, the USSR established satellite states in eastern europe. Previously, the soviets had directly annexed Ukraine, Belarus and the baltic states. Why didn't they continue to add more republics to the Soviet Union?
Well, it's worth noting that through the Winter War of 1940 the gained territory from Finland, having just annexed the Baltic states.
Also they did annex big portions of Eastern Poland(as part of the Ukrainian and Belorussian SSRs). Did you mean to ask why didn't they annex Poland or Hungary?
I took a Russian history course in last spring and my professor explained that Stalin did not have a real plan as far as what countries would come under the USSR's control but rather however far the Red Army would get would become part of the Soviet Union.
I can't speak to the Soviet leaders after him without being speculative so I won't.
This article might help you a bit.
http://www.bbc.co.uk/schools/gcsebitesize/history/mwh/ir2/sovietexpansionineasterneuroperev1.shtml