- The Soviet Empire never transformed into an assimilative empire and was always something of a post-war occupation state. If you look at other empires that managed to last longer, they almost always had an “absorbing” of the captive areas, adopting some of their cultural ideas, incorporating local elites into the state apparatus, etc. (think of the Romans in Greece, for example.) This didn’t really happen in Soviet member and client states and was more of a top-down promotion of particular individuals and parties.
- The existence of Poland. Soviet history really goes back to Russian Empire history, which includes the partitions of Poland and the numerous uprisings for independence during the 19th century. I don’t think it was ever realistic to think Poles would be happy in a Russian empire, and the continual existence of such a large, unhappy client state in the Soviet sphere seems like it was an inevitable weak spot. The Solidarity movement was key in bringing down the USSR, so that is essentially what happened.