End of the day, people preach on Communism because they simply can't accept that what they think is a great theory on paper always topples into horror in practice, but it's like some piece of architecture that looks nice on the board but the engineer tells you can't be built because the laws of physics simply don't permit it and the architect goes away grumbling and insisting how great it would be. To quote Frank Zappa, "Communism doesn't work because people like to own stuff.". Even if the holdouts who are so sure Communism would work if only we tried it in a place with Democracy and wealth already in place, it would inevitably lead to totalitarian rule, same as no manner of mucking about will allow you to make a suspension bridge longer than the tensile strength of the material permits.

The reasoning is simple, if in the next few election cycles we elected sufficient people to permit Communism via legal democratic amendments, and we still elected our leaders, you'd still have a near instant black market that could only be crushed by extreme measures. People would want things beyond their allotment, particularly since production would almost certainly drop off from lack of incentives. It wouldn't need to be big, it wouldn't even have to include material stolen from collective factories and fields, it could just be someone who operates their own backyard garden on their leisure time trading with others who do, but doing such a thing would have to be illegal and strictly enforced or it would become the norm. People would setup their own tiny gardens and cottages industries and even services like cleaning people's homes or mowing their lawns for them in exchange for things. Shortly after, the workers who have absolutely no incentive to produce will find every excuse to relax as much as possible at work, or miss work, to invest more energy into their home gardens, cottage industries, and service work. This doesn't even go into more extreme options that can and would happen, only the sorts of things civilized folk would disapprove of people being punished for and which would be nigh impossible to enforce without trampling any semblance of privacy and diverting large amounts of manpower to enforcing instead of producing. Doing that of course would further decrease total official production and increase demand for black market goods, thus increasing the incentive to do them and laying a great foundation for bribery and corruption.

None of that even requires extremes. Few people will feel guilty about accepting a basket of strawberries for looking the other way to a small neighborhood's secret gardening surpluses, home distilleries, or work pacts where Bob mows Kevin's Lawn in exchange for Kevin handing him a bushel of apples, on the simple capitalist concept that Bob kinda likes mowing and Kevin enjoys gardening. Wouldn't be long before someone was printing up home made 'favor tokens' for people to use to streamline this sort of exchange. If the various neo-commie Greens had their way, and we had various Utopias with 30 hour work weeks, an awful lot of us who like to be productive with our time would be, especially with so much of it. Jessica would tutor neighborhood kids after school, increasing the odds of them achieving higher status in whatever fashion is allowed, and Martha would be grateful for the help and insist her kids stayed around for a few minutes to help Jess with her chores and Kevin would make sure to bring by some apples in gratitude, and all the lazy bastards who make up a big portion of society would find themselves edged out, and Martha's students would probably rise to positions of power compared to the norm and be inclined to look the other way, and so on. "Communism doesn't work because people like to own stuff.", and any vaguely civil society believes people ought to benefit from doing work.

But all of this overlooks yet another reason why democratic communism doesn't work... because it is totally legal. You do not have to have a whole nation do it, it has always been legal for 5 or 6 people to get together and pool their resources, most families operate this way. It is totally legal for 50 or 60 people to do this, or 5000. It never happens, because if you get people who are willing to throw into the pool they will usually resent others siphoning their work. 5000 committed communists could simply get together and pool their money and labor, they cold easily collectively purchase any of the items or services they couldn't for whatever reason produce. They could buy a group insurance package, own factories and agri, all of that, and frankly if they were all committed to the idea do pretty well and attract converts. So the better question is why this doesn't happen, when their is ample opportunity and precedent for worker-owned factories and agricultural collectives. Free Market Democracy totally allows such things, and the only breaker is that such a society would not allow any punishment but exile for failure to comply with the mini-communist regime's rules. And the answer is simply that such concepts fall about off the drawing board because most of the people who want such a society aren't typically the productive sector of it. The fruits of their own labor don't seem to amount to much, so they want someone else's, for those who do get a big harvest, they will resent others taking all of it, and much of the middle will simply grow lazy without any incentive to work hard. Even those of us who by nature aren't much driven by compensation and like to produce and create and enjoy sharing our excess expect our compensation in the form of gratitude, and will cease to aid those who don't show it and repay it in some fashion and degree.

This is why so many of us always say communism invariably ends up with work being enforced at gunpoint and everyone getting an equal share of nothing, it simply is not compatible with a free society and the fact of the matter is democracy exists to protect freedoms, not as an end unto itself.