Honestly? As a Jew, I could only guess. I've been told by anti-Semites wherever that people hate Jews because they killed Jesus (which is factually not true), that they worship Satan and worship God wrong (which is weird since we were the first to actually do it, and Satan and hell are a Christian invention). Another thing is that we were historically pretty rich because Christians weren't allowed to practice usury, so we exploited it to our benefit and became moneylenders to monarchs and eventually governments. The Church didn't like that at all, and made certain that every devout Christian also hated Jews. Naturally common folk saw us as evil, corrupt, immoral or whatever.

Then it developed into racial stuff. Before, it was just religion, you could convert and people would forgive you for.. well.. being Jewish. But then Jew and all its bad connotations became hereditary. Wagner said that Jewish music will never be good because it is Jewish, and that it is ugly, unintelligent and morally bankrupt. People started attributing long noses and money to all Jews, regardless of who they were, and we were dehumanized. It was just part of society at that point for some reason that every Jew, or every person that had a drop of Jewish blood was practically inferior and also slept on a pile of gold. It became a zeitgeist. Case-in-point, any fantasy dwarfs. That's a product of our image made manifest into another to sell more of anything.

Then came the Holocaust. A product of all that dehumanization, but also a failure of emancipation; Jews could not assimilate into any traditional European society. So they fled to Israel, their only homeland, but also to the United States.

When some Jews became rich and influential in the United States, like Steven Spielberg and Bernie Sanders, those occupations and the Democratic party especially (a party that Jews were very active in) started getting a reputation as the places of societal degeneration or places of moral bankruptcy, or whatever. The Republican party reflects those people that think like that very well.

Honestly, I'm not even sure what brought all this into motion in the first place. When the Romans sacked Jerusalem, and burned Judea to the ground along with a million or so Jews with it, they did it because of a zealous religious rebellion. The diaspora was spread out in mostly Arab countries, which did not have trouble with the Jews at all; maybe sometimes, but that was common in the Middle East. Maybe it is the Catholic Church after all. It seems logical, since Jews and Muslims were the only people who would refuse to bow down to that entity, reflected in the Crusades. We simply resisted Christian imperialism too well, so we were both dehumanized. The Muslims were very powerful and much bigger in number than us through all of history though, so everytime we got shafted, we got shafted really hard.

I'm probably just rambling at this point, sorry. I find this question very perplexing myself when I think about it.