Three pretty big reasons that come to mind:
- The establishment, mostly supported by Jews, had their narrative of "brown people good, white people bad" backfire thanks to this conflict consisting of brown-skinned Palestinians fighting off pale-skinned Jews.
- The parallels between apartheid-era South Africa and Israel are extremely hard to avoid noticing, and Apartheid is practically viewed as the work of the devil among leftists.
- The fanatical devotion to Israel presented by evangelicals, neocons and RINOs ensures that some leftists will support Palestine purely out of spite towards conservatives.
Yeah, it's pretty blatant that this whole conflict is just an excuse to push the Palestinians out of Gaza into Egypt, and then not let them back in because they're technically not Israeli citizens. Therefore making them stateless (since Israel doesn't recognize the existence of a Palestinian state), and forcing Egypt to look after them. Egypt will then dump them into the hands of NGOs who will ferry them across the Med into Europe.
Coincidently, a lot of these NGOs that bring migrants from North Africa into Europe are run by dual Israeli citizens. Really makes you think...