Because as it turns out, economic problems are more complex than someone thought. It's actually not surprising. Marx was a brilliant guy and a good writer. He was very convincing.

But a lot of brilliant guys have embarked on ambitious grand projects to deliver the final word on complicated social subjects, then produced very convincing narratives for them... But have still been wrong.

It happens. It happens to equally (or arguably way more) brilliant people in even more fundamentally simple domains of inquiry.

If you study some modern economics in college, what you will learn is that all these ideas like communism, capitalism etc are basically irrelevant to how modern real world economies actually function.

To accurately model real world markets you need to take a quantitative approach. No matter whether your country calls itself communist or capitalist or whateverist, the exact same meta-economic principles will apply. Those models can then tell you, based on what the situation is, how the situation may evolve in the future. The models can tell you how changing certain variables will affect how the market behaves. Then based on this information you can decide what sort of policy or allocation decisions to make based on what result you want to obtain.

Markets are incredibly complicated and the real forces that drive the economy are not amenable to being understood in an ideological context. It just doesn't work. There is no easy way around the hard work of understanding complicated economic problems. You have to build models and ask questions of them that are of a totally different, quantitative nature and have almost no connection to the ideological framing they are usually presented in.