Marx is absolutely canon, one of the most important writers in history. I'm not sure how anyone can claim to understand the western tradition without Marx.
Bloom cared about the canon for its aesthetic value, and was quite skeptical about the whole notion that literature (in a narrow sense) could be relevant to social change. From that POV, the absence of Marx (or for that matter, of many other non-fiction authors, who are included in other "great works" lists but missing here) might be quite understandable.