Most of the time communism supplants a country's political system because the needs of the masses for food, shelter, and medical care are either unavailable or too expensive for them to attain. When the mass of people find it too difficult to exist and they see that the privileged or rich have everything they need their senses of justice are enflamed. The problem is that although in theory communism should work, people tend to look out for their own best interests and are not too caring about their neighbor. People generally are as altruistic as they can afford to be. Countries that are poverty stricken, exploited, and/or accustomed to tyranny are more subject to fall to communistic thinking. The problem is that they go from one extreme to another. From abject poverty to poverty and tyranny under Communism. They jump from the frying pan into the fire.
Communism ultimately fails because it does not consider that people have dreams of their own that they want to pursue regardless of how it affects their neighbors. If success or the dream of it is capped it leads to disillusionment and feelings of oppression. Free will is God's greatest gift to humankind. When free will is stifled under a certain system of government it is doomed to ultimate failure.
The end of this article on Encarta gives a good synopsis of communism and why it failed: