In Catholicism during medieval times, it was forbidden for Christians to lend money with interest (based on selective interpretation of the Bible). Jews would of course be exempt from that, so it was a nice workaround, because there was no ban on Christians lending money from Jews. This enabled Jews to get an early foothold in the finance industry before banking was even really a thing.

Combined with the already mentioned ban on Jews working in most professions in Christian countries well into early modern times (many places until the 19th century in fact), it was a pretty natural development that many Jews entered this profession.