Well for sure one of the reason is that even in non marxist communities they show up and write comments in a style that many people resonate with. I mean using citations, good grammar, appealing to logic, (seemingly) good argumentations etc.
This makes them look reasonable and even if you do not read all their sources, you might remember their comments and talking points in a pretty positive light.
I dont want to say this style of arguing is bad, but I think it gets valued higher than arguments based on intuition and emotion with a less "scientific" style, because that is what many people are taught when growing up, going to school etc.
Also I would like to say, in some cases this "showing up" is done in a way that feels invasive to (parts of) communties. Like an online version of Jehovas Witnesses.