well capitalism is based on wealth and communism is based on equal treatment for everyone. in communism everyone gets paid the same way. so if you are doing your work at a great pace and someone else is doing terrible, they will be paid the same, then people will begin to feel that they don't have to do anything but show up to work and they will be paid. this is one reason a communist government will not last long. capitalist governments however will last much longer because you get paid based on your performance.
In the real world, the only feasible and maintainable system is capitalism. With little regulation it is self sustaining and allows interchangement between classes, low prices, and pay based on performance. Its use of competition to drive down prices and keep up improvements is not found in communism and that alone is enough to put it ahead of the "if it aint broke dont fix it" system that communism perpetuates.
Communism has always failed. Capitalism is neither stable or humane and has also failed. All countries have embraced a system of government-constrained capitalism to keep the wealthy from destroying their own countries. So socialism, in varying degrees, has worked the best overall.