In one of my classes, my professor touched on it a bit, and so I emailed him for a little further depth. The time period context of this is medieval Europe. Here is what he said:
Jews engaged in money-lending in medieval Europe because (1) they'd effectively been eliminated from farming after the Roman Empire became Christian by laws forbidding them to own slaves (who were necessary for agricultural estates) and because, in the feudal system, even peasants had to swear Christian oaths to the Christian noble who taxed them; (2) they were eliminated from crafts and local commerce because these economic activities were organized into guilds that were defined in Christian terms; (3) they were eliminated from professions that required a university education (law, medicine, etc.) because medieval European universities (Bologna, Sorbonne, Heidelberg, Oxford, etc.) were controlled by the Catholic Church; (4) they were eliminated from government bureaucracies, since government was also defined in Christian terms; (5) the Catholic Church forbade Christians to charge interest on loans on grounds of Exodus 22:25 (which forbade usury/interest generally), but Judaism (on grounds that Deut. 23:19 forbade only charging interest "to thy brother") allowed Jews to charge interest on loans to non-Jews.
A good recent book on the Protestant Reformation is U. Rublack, Reformation Europe (BR305.3 .R83 2005).
[courtesy dr. green]