The obvious explanation for this is just the more general observation that most anarchists in the real world despise Marxists. In anarchist circles in private the discourse than ML's are a bunch of homophobic, transphobic, sex-worker-phobic, misogynistic red fash is very, very present, and honestly pretending otherwise is simply ignoring the obvious truth that becomes evident if you actually spend much time in read-world anarchist and Marxist circles, simply for the sake of preserving the appearance of an artificial, digital 'left unity' which neither has any bearing on actual organization nor does it provide a serious basis for any actual platform of organized socialist activity. We can get together for the same marches, social movements, or for forms of local mutual aid and aid for the homeless or refugees, but this does not ever really extend beyond that in my experience, and the reason is that anarchists have a fundamentally different conception of politics and organization to Marxists, and especially to MLs.