The "muh human nature" argument is a fallacy, you do realize that, yes? People are products of their environment, in Capitalism greed and selfishness are rewarded, so you think the way people act in Capitalism is natural for all economic systems, lmao.

I don't think that's the right reason, though it does touch upon one of the biggest reasons.

Communist projects have failed in no small part because of external interference from non-communist countries. Look at the US and their infamous "bringing democracy" around the world, for example.

But they've also failed not because of innate human nature, but because some people's nature is indeed what you describe. And unfortunately, violent revolutions have a tendency to make it very easy for those people take step in and fill power vacuums left in the wake of the former regime's demise. Even if the ideals of many of the boots on the ground in the revolution was entirely well-meaning, the leadership might not be, either from the start, or as the revolution goes on. That's why so many of the more famous communist regimes are incredibly authoritarian.

Name me one communist regime and I'll tell you why you're a fucking idiot and don't know the difference between communism and socialism.