Because being critical of religion is not an ideology. Atheism (since 99.9% of "satanists" are just edgy atheists) is not an ideology. When you're an edgy teen it may feel that way, but in reality you can't build a political and ideological identity on what is simply a rejection of one particular claim. But since they tried to make atheism into a group identity they needed to actually add a whole ideological framework around their atheism. Their ideological conflict with communist atheists becomes inevitable when they start to advocate for extreme individualism and Ayn Rand style sociopathic egoism. That is always going to clash with communism regardless of any shared belief in the non-existence of a god or gods.
Or to put it more simply: being an atheist is just not sufficient grounds to guarantee compatible world views between different people and groups. I'm an atheist but i have more in common politically and ideologically with communist Christians and Muslims than i do with libertarian atheists.