It wasn't just a matter of interest. Aaron of Lincoln produced some quite advanced mortgage instruments around the 12th century. Also, because the Jews weren't subject to feudal laws (they were the property of the king) they could move around and run businesses, at a time when there was very little int'l trade.
I read last year than as an alternative to having to ship gold from the Americas to Europe (and risk having it stolen by buccaneers or the English), it could be deposited with Jewish or Italian bankers in Surinam, who would issue you with an IOU which could be cashed in in Leghorn. Brilliant, but needs good logistics and a degree of trust....!