Communism in practice fails at a few things:
1) Opportunism, greed, and human cognitive failures. In theory, TS possible to evenly distribute the riches of a country to all its citizens. All it takes is a few people to start getting a bit greedy, and hen everyone else follows. Not only to people generally always want more than they have, they also really hate neighbours getting rich off their loss.
2) Incentives to innovation, invention, and entrepreneurship. Business competition and greed is a huge motivator for improving production efficiency, improving products, and making new things. Sure, communist society still has a push for innovation and invention, but this push comes either from trying to solve an existing problem (same incentive in capitalism), and/or government encouragement (but if you start rewarding some people more than others, that fails the "to each according to his needs" part of communism, see my first point). Rewarding individuals for exceptional work is inherently not communist.
3) Distribution of resources. Sure, its theoretically possible to have a really good centrally planned economy, but in practice we have yet to pull this off. Capitalism crowd-sources this task through auctions, price negotiation, and price signalling. Not perfect by any means, market failures do happen, but it has been better than any centrally planned economy.
The "Communist" countries failed hard at all of these problems. Nepotism and corruption was rampant, the attempt at central planning was a disaster, let alone central planning decisions that were purposefully bad (see: Ukrainian famine, also Mao and steelmaking). It might be possible to have a communist country, but it acted in good faith and were completely selfless.
Democratic capitalism has all kinds of failures, but it seems to be much more resistant to incompetent, or even malicious leaders.