Because there has, for a very long time, been a powerful anti-Zionist strain in the activist movement wing of the left. This is so widespread that a Jewish member of that movement took the trouble to write a large pamphlet (many pages) about how to recognize and fight anti-Semitism within those movements. aprilrosenblum.com/thep… Currently you can see this anti-Zionist strain in one of the organized BLM organizations as well.