Those who enjoy the wages of imperialism are more likely to have disdain for, or disinterest in, the complex struggles for national liberation in the periphery, which is dismissed as “the savage barbarism of the East,” in the choice words of Max Horkheimer.
This chauvinistic attitude has become so foundational to Western Marxism that theorists in this tradition often behave as if there were no need to actually study the history of socialist states in any serious manner. In fact, the attempt to do so is often looked upon with suspicion, as a sign that one might be a boor siding with the slaves, rather than a professional intellectual with a keen sense of what is worthy of scholarly inquiry.
- Gabriel Rockhill - From the introduction to Losurdo's Western Marxism
I think there's an element of western Marxists realising that the population they're in is so effectively propagandised against (see: cold war, red scare, etc.) that drawing some kind of delineation is seen as necessary to progress any kind of argument. It's a consequence of the victors writing the history books over here.
Any highlighting of western atrocities is always handwaved away as "whataboutism". The west more or less invented a logical fallacy purely to prevent fair comparison in this specific context, and it is seen as generally applicable despite the fact that in any other context such comparisons would be seen as valid. Western influencers generally seem to hold to a view that moral history started in 1917 (except, of course, when it comes to Palestine) and I think this is broadly seen as a way to try to "work around" it.