Why does socialism not work? There are many arguments that socialism doesn't work. It's becoming an increasingly popular topic as congresspeople such as Bernie Sanders and Alexandria Ocasio-Cortez push for policies that are considered to be socialist.

However, the articles about why socialism fails generally cite multiple different reasons, and there seems to be no clear argument.

The only reason why something would fail practically when it should work theoretically is that it is making incorrect assumptions about the world.

What assumptions does socialism make in order to work theoretically, and why aren't those assumptions accurate reflections of the world?

(Please note that this question takes the standpoint that there are incorrect assumptions in the theoretical basis of socialism, and is trying to identify weak points in this theoretical basis. It may be true that socialism does work, or that there's a version of socialism that works - this question is specifically asking about what parts might not work. Similar questions may be asked about capitalism, or other economic theories.)
Socialism doesn't work because an elite does not like to give up their privileges easily. After it took a wave of bourgeous revolution over a number of centuries to force the aristocracy to give up their powers and to enter a power-sharing agreement with the bourgeois which led to the decline of feudalism. 

Marx suggested that this movement would not stop with the bourgeois, but would then go on to animate the so called third estate.  This indeed did occur over the 20C, but in the final analysis, not successfully and this has led to some people to say that Socialism does not work. The other side of this thesis is that it was simply strangled at birth, and this is what the Trumans policy of containment amounted to.