I believe there are several reasons:
1. It is an easy way to express one’s hatred for America.
2. Israel is viewed as an outpost of colonialism , and an active practitioner of it.
3. Israel is a western nation, and hence can be judged by the left. Israel is not protected by cultural relativism, as the Arabs are.
4. Leftist Christian churches can escape any lingering guilt about the Holocaust, by turning Israel into a villain. Some leftist churches hate Israel because they think this will help protect their members in the holy land- in other words they feel threatened.
5. Ferocious Muslim hatred of Israel and the Jews reinforces the natural cowardice of many on the left who go along with the Muslims to stay out of their line of fire.
6. Jewish leftists are prominent in the anti-Israel movement. This opens the floodgates for everybody else.
7. Israel is attacked because the secular left is appalled by the influence of religious settlers and their biblical connections to the land of Israel, and by the support for Israel by evangelical Christians, and Christian Zionists...