Well this is a complicated question. The article I posted shows some of the differences in terms of forming Jewish identities, so much so that American Jews and Israeli Jews are distinct groups, with some degree of overlap, but with fundamental differences in how they see the world.

To really answer your question, we would have to start with the interwar period. Antisemitism was at an all-time high in America. Newly immigrated Jews had some trouble operating in a largely Protestant society, at which point they formed largely Jewish societies (like Felix Adler's Ethical Culture Movement, which ran schools in NY and NJ) and the American Jewish Congress. Though many of them were initially Republicans, by 1916 more than 50% voted for Wilson, the Democratic candidate.

That pattern continued with the New Deal Coalition, with larger and larger proportions of Jewish Americans voting for Democrats, as the Northeast Democrats that led the party were more open to civil rights discussions than the Southern wing or Republicans. Social justice became an essential part of American Jewish identity. That forms the basis for continued support. A significant portion of American Jews, who've always relied on the plurality to ensure they have a voice, were/are weary of the explicit ethno-nationalism of Israel, which is immediately in conflict with the pluralism which afforded them a voice in America. So what is antisemitism to the American Jew is different than what antisemitism is to the Israeli Jew.

One of my best friends is Jewish, and he's the most anti-Zionist person I've ever met. His Jewish identity was formed by his American experience, and was cemented by his trips to Israel. That's the essence of the division, as though conservative Jews are sizable, they remain a minority of American Jews (something like 20%), and even then, you'll find a wide variety of opinion about the differences between American and Israeli Jewish experiences.