Ask Lemmygrad

Others already gave good generalized answers. I would like to make a mention of western marxism specifically because in english-speaking circles, it's where you're liable to find a lot of ultra-style positions coming from.

In essence, the west (but mostly intensely, the US) put a stigma on any kind of support for communism (via Red Scare, etc.). This helped corral people into a very narrow and disarmed form of marxism, where they say things like, "I believe in communism, but it has never been properly tried." Or, "I believe in communism, but those socialist states (USSR, China, Vietnam, Cuba, etc.) are revisionist because they deviated from true marxist practice." This allows them to land in a more palatable and non-threatening ideological place where they cling to the idea of support for marxism in theory, but in practice, they take the side of the empire in disavowing the leadership of socialist projects (even if they don't disavow them for the exact same reasons in every case). There's a certain irony to it in this form because they are themselves capitulating in order to be able to "safely" say anything about marxism, but are accusing actual practicing marxists of being the ones who capitulated and failed.

But bottom line is, they end up throwing their international comrades under the bus to be able to push a theoretical view that doesn't go anywhere near practical, applied liberation. There is plenty to learn from international socialist projects that have succeeded and continue to succeed in many ways. Not because they are magic, but because they put the theory into practice, they developed it beyond what marx had to say about it (ex: ML), and they produced many improvements to quality of life for their people with it. There's not a lot to learn about the subject from academics who never put theory into practice, unless they're trying to pass along knowledge they learned from people who did.

For a rough comparison, imagine if somebody insisted that space flight has never been tried and the people who have tried have deviated from real rocket science theory, and that's why sometimes accidents and loss of life have happened. Rather than the fact that space flight is incredibly dangerous and can't be made risk-free.