score:10
The answer is simple. In the parts of Imperial Russia that became the USSR, the Communist Party had managed to achieve sufficient control, and was determined to form a union. The parts that became independent had strong nationalist movements which won their conflicts with the local communist movements.
In Austria-Hungary, the nationalist movements won everywhere. The unions of Czechoslovakia and the Kingdom of Croats, Serbs and Slovenes (later Yugoslavia) were formed more or less willingly by groups that feared they were too small to form successful nation-states.
Upvote:3
There was an important difference between the two empires. In the case of the Soviet Union, there was one country, Russia, with 50% or more of the people of the whole Union. The next two, Ukraine, and Byelorussia, had about 15%. The remaining 30%-35% were divided among 13 Republics, averaging 2%-3% each. Russia was so much larger (in population) than the others, so the most of the smaller states were easily subjugated.
In the Austro-Hungarian Empire, the (Germanic) Austrians were a minority. I don't have most of the 1910 population figures but today's are indicative: Austria, 9 million, Hungary, 10 million, Czech Republic, 11 million, "Yugoslavia" (Croatia, Dalmatia, Bosnia, Slovenia,), 10 million, Slovakia, 5 million, Polish (in 1910 Austrian Empire), 5 million, Romanian, (in 1910 Austrian Empire), 3 million. Put another way, the "core" Austrian part of the Empire was "one of many," not "dominant" as in the case of Russia.