Because the Jews are, on the average, probably smarter than most of the rest of us, and because they were not allowed to own land in Medieval Europe and because they weren't forbidden by The Church to lend money at interest, they became the traders and bankers of Medieval Europe. Because they (this also applies to Christians once they got the hang of it) could trust each others' word they effectively invented Capitalism and also became the moneylenders to impecunious kings. As traders many got visibly wealthy without being rulers. All of this made them hated by the Christian and not-so-Christian ruling classes who from time to time got out of their indebtedness to Jews by inciting pogroms among the serfs and townsmen and by expelling the Jews and expropriating them. People never feel charitable toward their creditors and the less affluent have tended to resent the more affluent throughout history.
Ruling elites have always hated and resented the commercial class because they have to depend on them.
By the way, Arabs will never become great in the commercial world because of the non-Christian inability to trust each other for anything outside of the family-clan. That is an Asian phenomenon and finds its most concentrated manifestation among Moslems and particularly among Arabs.