Thomas Sowell says it is because the Jews are middlemen minorities, similar to the overseas Chinese. As such, they are often in the position of creditor (”Shylock”) demanding his “pound of flesh,” and landlord (Snidely Whiplash in the Rocky & Bullwinkle Show), about to evict the poor tenant.
http://www.hoover.org/research/anti-semitism-generic
A stereotype Goebbels and Co, used in the movie “Jud Suss.”