Even if a communist can colloquially describe themselves as being on the left, there’s a distinction between communism and “the left.” This is implied right in the title of Lenin’s Left-Wing Communism: an Infantile Disorder. Whereas the left, a big tent term for a myriad of incompatible ideologies, aims merely to act as an opposition towards the present order for the sake of it, communists have a coherent vision for how to defeat the system: by advancing history’s development to the next stage. The left, because of its lack of commitment to that central Marxist goal, naturally takes on an opportunistic role. Because when you want only to build a movement as an end in itself, rather than use this movement as a means for defeating the system, you become nothing more than an actor who benefits from discontent without helping solve the problems behind that discontent.
I think what the mixup is is that you are viewing things in terms of conflict as opposed to the further development of the productive forces of society. Slavery and Feudalism were overthrown because they could no longer satisfy the needs of society, and so the developmental forces had to evolve.