Economically, communism fails in practice because a society with no government (agency to protect rights), no money (no indirect trade, only barter), and no wage labor (no specialization under a division of labor) is a terrible idea.
Then why do people continue to support it?
“When, at the age of twelve, at the time of the Russian revolution, I first heard the Communist principle that Man must exist for the sake of the State, I perceived that this was the essential issue, that this principle was evil, and that it could lead to nothing but evil, regardless of any methods, details, decrees, policies, promises and pious platitudes. This was the reason for my opposition to Communism then—and it is my reason now. I am still a little astonished, at times, that too many adult Americans do not understand the nature of the fight against Communism as clearly as I understood it at the age of twelve: they continue to believe that only Communist methods are evil, while Communist ideals are noble. All the victories of Communism since the year 1917 are due to that particular belief among the men who are still free.” – Ayn Rand, quoted in the forward to her novel, We The Living.