Imagine two tribes.
One is Communist. The other capitalist.
In capitalist tribe, certain person makes spears. One day, that person discovers new way to make spears much faster and easier. Due to being selfish, that person keeps his knowledge a secret. That way, he earns more than others in tribe.
In Communist tribe, certain person also makes spears. One day, that person discovers new way to make spears much faster and easier. Due to being raised in Communist values, person shares his knowledge with everyone. Now everyone in Communist tribe makes spears much faster.
As a result, Communist tribe has much more spears.
Now compare this to today's societies.
In Capitalist society, rich man doesnt want to pay for education of the poor people. That way, the rich man keeps more money for himself.
In Communist society, rich man pays for education of the poor people, and society ends up having more educated people, more scientists, more inventors, and less crime.Entire society profits.
So in short, Capitalism is about profit for individual, where Communism is about profit for society as a whole.
This is why Communist societies always win. Its simply irrational to assume that individual's profit is more important than profit of society as a whole.