Every time communism was tried without the backing of state power, it failed. Whenever it was tried with the backing of state power, it failed after widespread bloodletting, starvation, and tyranny.

There are several prominent theories about why communism is an untenable way of organizing society, most notably by Hayek and Popper, the former providing a literal road map of how communism was bound to fail. Of course, many people have theories about how society can and cannot be organized, and it is almost impossible to run controlled experiments on real populations to see what might work, which is why the die-hard faithful will point to the failures of Soviet Russia and Maoist China as just two data points that don't sufficiently prove communism is a failed ideology.

In fact, communistic societies have been tried hundreds of times over the last two centuries, mostly in the form of small communities in rural North America. The overwhelming majority of them have failed within 18 months. Some towns, such as Utopia, Ohio, and New Harmony, Indiana were second-generation attempts at such societies, presumably with all the lessons learned in prior failures. (FWIW, a good number of refugees from these societies became committed individualists and believers in the virtues of private property.) The relatively few voluntary communistic experiments that lasted more than several years ended up dramatically diluting their communal ownership of property, as one type of communal asset after another suffered from the tragedy of the commons. Israeli kibbutzim, one of the more successful attempts at a communal lifestyle, have since largely adopted differential wages and private property, looking more like company towns than socialist utopias (this Community Profile shows a typical example).

Dozens of Intentional Communities exist today around the world, many of them based on socialist/communist governance, but none of these communities have attracted more than a tiny fraction of the populations in their respective countries, and they continue to fail at much higher rates than other types of towns created from scratch. One would think that small societies composed entirely of self-selected true believers would give such social experiments a significant survival advantage. But for some reason, all that individual commitment consistently fails to translate into collective success.

Attempts to implement communism via state power have generally, notoriously, been totalitarian bloodbaths followed by extreme privation. In the 20th Century, 94 million people have perished in failed, state-sponsored implementations of communism.

Unrepentant Marxists and naive youth can complain all they want that Soviet or Chinese communism wasn't really communism. However, when you study all of the attempts mentioned above, both voluntary and those supported with state power, one can only deny that all these failures are indictments of communism via the No true Scotsman fallacy. To a reasonable person, when every large-scale implementation of a governance practice invariably leads to escalating violence and coercion to make it work, that should tell you something about the compatibility of that governance practice with the nature of the people one is trying to govern. In other words, if communism is incompatible with how people actually behave, blaming human nature does not absolve communism of its unworkability.

In sum, communism is a failed ideology because all attempts to implement it either voluntarily or by force have conformed far more closely to the theories predicting its failure than those predicting its workability.