Because it was dominated by the Soviet Union and had communist government.
The term "Iron Curtain" dates from after the WW 2 and Nazi era. The eastern European countries behind the so-called curtain were dominated by the Soviet Union.
containment
germany and soviet union
During the cold war era each European country was classified as part of either Free Western Europe, or of Communist Eastern Europe which was mostly dominated by the Soviet Union until the Soviet Union broke up.
The Soviet Union was ruled by the Communist and the Governmental powers of that time.
containment
You are probably looking for Iron Curtain, although Warsaw Pact would also work, or satellite nations, or puppet governments.
Germany invaded the Soviet Union.
Group of Soviet Forces in Germany ended in 1988.
Group of Soviet Forces in Germany was created in 1949.
Containment.