It isn't to the faults of humanity. It is to the faults of capitalism. Every human ultimately acts with self-interest. Capitalism causes this self-interest to be manifested as 'selfishness' and greed. For if a system works entirely on obtaining and circulating meaningless bits of paper - people are ultimately going to strive for those bits of paper. Socialism, and ultimately, communism could work and has worked. I feel it could work in the Marxist sense, and it has worked in the Anarchist sense. The problem with the USSR was Lenin's interpretation of Marxist philosophy, and with China it was the empowerment of the agrarian societies over the proletariat. For communism to work - you need communists. How can you possibly expect communism to work when it is enforced? Especially since enforcing communism makes it no longer communism. If you have any questions, feel free to ask.