The main reason is that Israel is materially supported by the West, so Westerners feel morally responsible for what it does.

It has little to do with whether the perpetrators are Jewish or not[1]. There were gigantic protests against the Iraq war, whose main perpetrators (e.g. Bush) were not Jewish.

1: I edited this from "nothing" to "little". I concede it might have something to do with anti-Semitism, because there is some non-zero group of people whose opposition to Israel is purely motivated by anti-Semitism, but I don't get the sense that they're the majority, at least among Westerners.