Why were Jews the money lenders in Christian Europe?

May or not be a comment; but to elaborate on Peter Flom’s answer.

In Byzantium, there was no comparable ban on interest. So Jews were not the money lenders in Orthodox Christian Europe. Jews were driven to do different shit-work that Christians there wouldn’t touch.

Literally shit-work. Jews were predominantly tanners in Byzantium. Before the industrial revolution, tanning involved the use of animal dung to soften animal hides.

Hence the references in 14th century Greek vernacular poems:

• The Book of Birds’ description of the pelican’s bill: “You satchel of Moyshe’s full of dogshit.” (αποθηκάριν του Μωσέ γιομάτον σκυλινέαν)
• The Entertaining Tale of Quadrupeds’ description of the boar: “The Jew stinks; and his satchel’s just as smelly” (Εβραίος όζει και βρωμεί και όλη του η θήκη)

Cf. Benjamin of Tudela, a Jewish traveller who visited Byzantium in the 12th century. He may have gotten a bit mixed up:

For their condition is very low, and there is much hatred against them, which is fostered by the tanners, who throw out their dirty water in the streets before the doors of the Jewish houses and defile the Jews’ quarter. So the Greeks hate the Jews, good and bad alike, and subject them to great oppression, and beat them in the streets, and in every way treat them with rigour. Yet the Jews are rich and good, kindly and charitable, and bear their lot with cheerfulness.

The tanners fostering the hatred are in fact Jewish.

Much more in Byzantine Jewry in the Mediterranean Economy, and several other sources.