The same reason there are white supremist and neo Nazi parades now. People basically explained it in answers above. Antisemitism has always been around as far as I can tell. Always, throughout time, more heavily in some places than others and also worse during various times in history. People think the Nazis were the worst, but during the Pogroms in Russia it was no cake walk for the Jewish people. There were signs in shop windows saying “no dogs or Jews” in some parts of America less than 100 years ago. Jews were accused of owning everything, even money, because it was not unusual for Jewish people to be in the banking business, and in hard times, if they held the note on your property and you couldn’t pay, you lost your house to the bank. There is an old joke that Jews don’t ski they own the mountain. It isn’t true that Jews own the majority of property, how can it be, we are 2% of the US population and less than one quarter of one percent of the world population, we are a spec in the world population, but still we are very often the scapegoats for people’s whoas. It’s a tradition.
Add in fear of comunism, and economic difficulties in America. Remember, Hitler was voted in, Germany was having difficult economic times also. Germany was modern, a democracy, it would see unthinkable what actually happened under Nazi Germany. It isn’t that the Germans are so different. Growing up Jewish we are taught if it can happen in Germany it can happen anywhere. It was not a backwards country, it was a country that pursued science, was sophisticated in many ways. My point is, it shouldn’t be so surprising there were Nazi sympathizers in America. Henry Ford (the auto guy) published The International Jew basically saying getting rid of the Jews would solve a lot of problems. Back in the day Jews did not buy Ford they bought Chrysler.