Because it wasn't socialism. It was a centrally planned capitalist economy. There were still commodity production, accumulation of value and a State that regulates value. The only way in which USSR and the Eastern Block was socialist is the way they called themselves. If they called themselves aliens, it would make about the same sense. Even Lenin called the system "State Capitalism" in his works. Socialism in the classic Marxist sense -- which is the exact same thing as communism, communism/socialism divide nonsense comes from Stalin and makes no sense -- is the society after capitalist economy collapses, and the natural force that makes capitalism possible is value generation. It's simply by-definition not possible to talk about socialism in a society that still has value (money), the division of labor (job), class (worker vs employer) and state (a body that regulates trade which is integral to capitalism). Marxism is above all materialism and ideologies like Marxism-Leninism is the most anti-Marxist thing imaginable.