1. In ancient times, they were monotheists surrounded by polytheists. Their refusal to worship the gods of their overlords was considered a threat to the state because it risked divine retribution. This led to persecution by Babylon and the Seleucid Empire, and the same basic logic was behind the persecution of Early Christianity (which would’ve been regarded as just another Jewish sect for the first century or so).

2. The blood libel. This antisemitic conspiracy theory goes all the way back to ancient times. Greek and Egyptian writers claimed Jews abducted gentiles, sacrificed them in the Temple, and ate them.

During the persecution of Early Christianity, these tropes resurfaced with the claim that Christians stole Roman children and ate them and drank their blood as part of Communion. But once Christianity became the dominant religion in Europe, this screed was retooled to target Jews. Instead of Christians stealing Roman children, it became Jews stealing Christian children. And instead of cannibalism as part of Communion, it became cannibalism as part of Passover.

Similar conspiracies cropped up during the Middle Ages, like the idea Jews were behind the Black Death because it didn’t hit their communities as hard.

3. Religious differences with monotheistic majorities. Christian bigotry towards Jews was rooted in their refusal to accept Jesus as the Messiah and a perception of Jews as “Christ-killers” (even though it was the Roman Empire who executed him). Muslim bigotry was driven by their refusal to accept Mohammed as a divinely inspired prophet and convert to Islam.

4. Medieval laws limited the trades Jews could practice and the property they could own, leading them to gravitate toward professions like medicine, law, and banking. Their proficiency at the latter led them to be stereotyped as greedy.

This is what led to the whole “Jewish elites secretly run the world” conspiracy theory, which was popularized by The Protocols of the Elders of Zion. This was a document secret police in Tsarist Russia fabricated for use in antisemitic propaganda. You can trace a lot of modern antisemitism back to this forgery.

5. Jews retained their own culture and ethnic identity, leading them to be regarded as outsiders more loyal to their own than to their government. Being such an obvious “other” made them the perfect scapegoat for whenever anything went wrong.

You can see examples of this from the conspiracies about them causing the Black Death all the way up to them being blamed for Germany losing World War One.

This intertwines with the last point in that rulers would often scapegoat them as a way to seize their wealth.