> In general people tend to work on greed and fear.
I don't think that's an accurate depiction of humans. People work either: (1) they want money, and/or (2) they want something to do during their waking hours.
Even if I had enough money that I never had to work the rest of my life, I still would definitely work, because I enjoy work, and want to do something meaningful (from my perspective) with my life. I would most likely be a pro bono computer science researcher, and might maybe join a PhD program -- if I get to pick what I research on, which might be possible since I don't need grant money, and would cost the school zilch.
But basically, my point is that what motivates me (and most people) to work is either: (1) the love of work, or (2) the need for money.
Absolutely not greed and fear, as you say.
Now with regard to communist societies, I think the biggest problem is the utter lack of freedom. Maybe I'm wrong, but I have a strong feeling that the epic failure of communist economies like the Soviet Union or pre-1990 India could be directly attributable to heavy regulation of businesses. They made it hard to start and run a business.
Now I'm sure over 90% people will never start register a company in their lives (maybe that percentage is reducing now), but making it hard for 10% that do start & run businesses could be the direct cause for the failure of these large economies. For example, India, until 1990 followed Soviet Union-inspired policies, and one of the biggest consequences of this was insane red tape know as "License Raj"[1] wherein you couldn't start a business without approval from a dozen corrupt government agencies, and even if you did succeed in starting one, you were subject to an overwhelming number of regulations that made it insanely hard for you to run your business.
Why do I think making it hard for 10% to run business is the cause of their economic failure? Because it prevented any sort of free market capitalistic competition. Everything was run by a bunch of non-business-minded bureaucrats who predictably did a bad job at it. Restricting private businesses and especially competition prevented an increase in production efficiency, and made the economy as a whole less efficient. A lot of these factors and other factors rolled in to one, led to the hour long lines for milk and butter.
More fundamentally, regulations erodes away your freedom. If you want to do something -- and starting a business or selling something is doing something -- if the government steps in and says "no you can't do that without my approval" that is an insane Mafia-protection-racket-esque interference in your personal freedom. You're just trying to make something and sell it! Nothing wrong with that! People have been doing this for millennia! Why do I need to listen to you, follow your little rules, or pay you a fee, to exercise my personal freedom?
Regulations take away freedom. And enough of it destroys economies. End of story.
[1] https://en.wikipedia.org/wiki/Licence_Raj