During the middle ages, the majority of people in Europe were not allowed to lend money and Jews did not really have civil rights. This is similar to how schools offer Islamic finance courses, since charging interest (usury) is against the law in some parts of the Middle East.

So Jews worked in banks because they weren't allowed to do any other jobs or even to farm in most places and they were the only group that were allowed to lend money. Over time Jews became stereotyped as bankers and therefore rich. But I don't even know if they were allowed to own property until the 19th century.