In my personal studies about Positive Psychology, I came across a very good explanation in Tal Ben-Shahar’s book “The Pursuit of PERFECT“. The key to answer the question why socialism did fail lies in an assumption about human nature. Capitalism has a constrained vision of human nature. Capitalists believe that human nature and in particular everyone’s self-interest cannot be changed. It is better to accept this fact and leverage people’s self-interest for the common good. Utopianism assumes that human nature is unconstrained and therefore can be changed. Basic human instincts can be altered and self-interest can be replaced by altruism. Eventually, one could create a society made up of superhuman beings being in control of their human nature.
Socialism was founded on a wrong assumption. People did not change their human nature. We, in East Germany, maintained our self-interest. On higher political levels this led to misappropriation of resources and in everyday life self-interest decided about the allocation of consumer goods. In the end, it was also self-interest or human nature that led to the Change. See also previous posts about the drivers for change (Freedom and Material Desire).