The main argument for Communism is that everyone can be equal, irregardless of the circumstance.
The main argument for capitalism is individual freedom, and the ability to do what you want (excluding dumb things like murder and bombing your school).
Psychologically, people who get rewards consistently will only exert minimum effort to get the reward. A child given $10 for mowing the lawn, irregardless of quality, will do a shabby job.
A worker paid $100 a day will do minimal work required to get that $100.
What controls are there under Communism to ensure everyone strives to do their best? What controls are there to prevent "mindless hard work" (ie a man working 100 hours a week without achieving anything), and encouraging "quality over quantity" (a man working 2 hours a day, but achieving a lot).
And if so, won't those controls ultimately compromise equality, and lead to yet another rise in capitalism?
This is one of the reasons for Soviet and Chinese inefficiency; because state controlled factories paid everyone the same irregardless of their productivity or efficiency, it simply means they will do a poor job (my parents did just that in the 70s).