There would be several reasons, not that I am speaking for the other poster.
The first s that Marxism is the product of one mind (well, two if you count Engels). But more than that, there is no such thing as “Capitalism”. There is no such theory. It’s not officially written down anywhere and there aren’t any proponents for Capitalism per se. There are free markets, and in free markets capital may be allowed to accomplish certain things with the legal permission of government. And there are many supporters of free market in a wide array of ways, with somewhat different views on how they ought to function, and none of whom are even consistent with their own selves.
Free markets are an ecosystem. They don’t necessarily give you what you want, but they generally give you what you ask for - and they give it to you good and hard if you don’t ask for the right thing. But they are effectively extensions of the actions and decisions of millions of living creatures Communism is always a specific theory expounded by a certain person, and as Steely Dan Fan demonstrates the theory can never be tarnished by reality in the mind of the believer. But free markets are the reality; the theories come afterward only to describe and can never completely do so. Like an ecosystem, learning more about it simply opens up new questions, and the more you interact with it the more it changes in response to your choices.
Because Marx’s writings rely on a rather simple-minded sleight-of-hand. He proudly declares that his utopian vision, which was completely derivative, was based in science. He then proceeds to effectively declare himself a prophet of the future, more or less, and that he can scientifically derive what will happen. His economics are farcical and his understanding of economics deeply flawed, so even his reasoning isn’t very good. But his basic problem is that he has nothing else. He declares it inevitable - but it doesn’t seem to be.
But even leaving that aside, Marxism requires that human beings will (A) Kill all everyone above the proletariat, and then (B) Spontaneously organize itself into a global community where life will be perfect, all resources perfectly allocated, and no hard choices need be made. This is why Marxism isn’t the answer: Marxism can only work if all the problems have already been worked out. Marx has nothing that early utopians did not have, except that he was even more successful in spreading pseudoscientific nonsense.
This is why I object so strongly to people who applaud Marx as an “economist”. The man was a cult leader who presumed to handwave all economic problems away, and his later behavior presiding over the Internationale foretold the nastiness that Communists would soon be working up. Mass purges, irrational party dogmaticism, and double-dealing to preserve personal power were part and parcel of Communism well before it ruled nations.
And, for that matter, this is why Steely Dan Fan doesn’t understand Communism despite his shallow defense of it. Actual Communists were completely incapable of making Marx’s vision work because it was a fantasy and not an actual coherent plan, or even a concrete goal. No matter how dedicated the disciple, you can’t turn lead into gold even though your alchemical recipe claims to be flawless. The only way to approximate it was to force society into the Marxist mold and hope it would somehow work itself out.
It didn’t, and that won’t change.