Originally Posted by
Machismo
That's rather simple, people are terrible fucking beings, who have a tendency to be authoritarian, and oppress others. The problem with communism is that it has no solid mechanism in which to deal with observational outliers within a society, while at the same time, promoting corruption, as well as complacency. Communism on any large scale requires that humans not be corrupt, and not seek to rule over others.
There's also the issue of how people seek to implement communism, which is through heavy-handed acts, and significant violence. Communism really only works, when it's voluntary, and on very small scales. Otherwise, you get skidmarks like Stalin.
Communism isn't evil, humans are.