While I am no scholar on the subject, I think four issues come into play with the hatred of the Jews. Remember also, that hatred of the Jews has been around for about 1900 years, it did not start with Hitler, he just capitalized on it.
First, anti-Semiticism got its start about 1900 years ago with the belief that Jews were responsible for the death of Jesus Christ. Many interpret the bible as showing the Jews were a force against the church and tormentors of the church’s founding fathers. This was a bad thing to be early in the first millennia when the church was all-powerful. These anti-Semitic feelings began here with the church and were particularly strong in Europe.
Second, the Jews did not change their culture. They stuck to their religion and sub-culture so the never integrated into the larger societies they found themselves living in. This could cause problems because the world of 100 AD to 1970 AD did not value diversity as much as we do now. Remember, until recently people burned people for witchcraft and all kinds of other stupid reasons; being different sometimes had dire consequences. This reinforced the anti-Semitic feelings that already existed in Europe and caused the hate to grow stronger over time.
Third, since the Jews were relatively few in number but a large enough group to be noticed. Basically, they had no political power. So basically, once the hatred of the Jews started they were never able to marshal enough strength to overcome it, they always just stayed locked in this permanent second-class citizen status. They were never destroyed but never accepted.
Fourth, the Jews made great scapegoats. If it is the Middle Ages and people are dropping like flies because of the Black Death, nobles would blame the Jews for poisoning the wells. After the Jews were killed off in an area, the people would split up their stuff. Basically, hated and weak are a bad combination.
I wrote this quickly I hope it makes some sense. Basically, the hate started with the church, the fact that Jews do not change their culture to fit their surroundings increased the hate. The Jews were never big enough to protect themselves, and sometimes greedy nobles needed someone to blame to focus the local populations anger and the Jews were a convenient target.
Terry