The correct question would be: Why did the widespread antisemitism dwindle so much after WWII? Because of the Holocaust and biology, the weaird unscientific theories around at the turn of the century are now relegated to the fringe lunatics.
But before that, the history of hating Jews goes back to the Roman Empire becoming Christian.
Some points have already been mentioned: the Christ-killer thing that was propagated by the Catholic Church (Hitler got the idea for the yellow stars from the yellow hats and similar jews had been forced to wear in the Middle Ages; the name “ghetto” comes from the iron pourers quarter in Rome where Jews had to live because they shouldn’t live elsewhere…)
The money-lending things meant that people were angry at owing so much money, and killing them during a Progrom was a convenient way of getting rid of your debts.
And the whole stranger thing making them convenient scapegoats.
A lot of prejudice and hate fed on itself: in the Middle Ages, Jews could neither own land (and farm it) nor join a trade union and learn a proper craft. So the only jobs left for them was lending money and becoming traders. Both jobs were they couldn’t blend into the normal population, thus keeping them outsiders.
The 19th century just added a scientifc layer to the whole thing, when science was starting and a lot of quack theories were around. I have leaved through scores of booklets about the “Judenfrage” (Jewish question) from the turn of the century at work, and the big question (both from German and Jewish writers) was whether Jews could assimilate into society, whether they should be allowed to, if they should start their own state (see Herzls zionism).
Many educated Jews were only nominally practisising reform Jews or atheists and considered themselves as integrated being doctors and lawyers and similar. They understood the concern about the “backwards” Eastern Europe Orthodox Jews with their weird clothing and hairstyle, and if those people could integrate into normal society or be a drain. The weird race beliefs were on top.
And in many places like Tsarist Russia, anti-semitism was fuelled from the state because scapegoats were helpful. If the common farmer was angry at the Tsar for taking too much taxes, then a leaflet about the money-hoarding Jews was distributed, a Progrom took place, and the Tsar could sleep safely again.
This mechanism is still at work today, only with other groups as scapegoats.
Psychologial tests have shown that in order to unite a group, the best mechanism is not averting a danger to everybody or building a common project, but hating a common enemy. Sadly, this is what the tests show. Therefore, it’s so often exploited by populists.
Oh, and Paul, Americans for a long time didn’t believe that
At the end of the 19th century, Irish and Italians were hated and shunned in the US. Anti-black laws were also based on spurious scientific evidence, yet they endured till the 60s. Native Americans only got citizens right in the 1970s with the AIM.
And in the 1930s and 40s, there were signs in swimming pools on the East Coast that “Jews, Negroes and dogs” weren’t allowed. Certain clubs wouldn’t permit people of the wrong descent to join.
Even during the Holocaust, the US still blocked Jews from taking refugee there, because they didn’t want to become flooded with them.