They were rich. They were also very secluded, with their own religion and culture. But the most important thing is, they were rich and influential.
That didn’t sit well with the war torn, piss poor Germans living in Germany after WWI and the Europe’s Versailles treaty that was nothing more than a very, very disproportional retribution for the WWI. Germany was proclaimed as the only one to blame and got basically destroyed.
Anti- Semitism in Germany wasn’t a new thing; it has existed way before the wars, but it was much subtle and more benign. It exploded in WWII.
Hitler used what has already existed and turned it into his own benefit; the rich, socially stable Jews were already looked a bit bellow the eye. In a time of enormous inflation and national embarrassment, when people barely had anything to eat, Jews started standing out much more.
Soon enough, they became the main problem, the main reason for the collapse of once wonderful Deutschland. People were angry, disappointed and hungry, Jews had what they needed, and not only that, but Jews started looking like they took what belongs to the Germans.
The people were angry, and Hitler needed to channel their anger in some direction in order to control them. He thus channeled it towards the isolated, elitist-looking Jews that had all the good stuff.
It was a part of a grander scheme; in the times of massive tragedy, people needed someone to tell them it’s not their fault for being in such a miserable situation, that they are the victims, and that someone else is to blame. It’s very easy to blame your misery on someone who has it better than you. They were also the foreigners, the non-Germans. All non-Germans were massacred in Germany, Romani, Slavs, gays, mentally and physically ill all needed to be removed in order to create a new, stronger Germany that will show the Europe its real power. Jews were the first that had to go.
Another thing is that they were very educated, mostly doctors, lawyers, scholars, professors etc. Educated people are a threat to an authoritarian regime.