I'll expand on Joe's answer if you don't mind.

As most of us all know, its human nature to want more than we're given. Marxism is based on the fact that everyone is equal, which would mean that everyone is given the same amount of everything. If there was no greed in the world, it would work, but until then, there will always be those few people who just NEED to get more of everything. More rights, more money, more power. That little unsettling fact of human nature is why in theory, its the best, but in practice, it shall never work.