The same incentive you'd have to do something for someone without receiving payment - a desire to help. I'd imagine the fact that people in that time had stronger familial ties to others in their village also helped increase payback rates.
And both religious created a way to indirectly pay interest by turning it into a business partnership.
i.e. when you are helping someone (a friend, relative) you don't pay interest. But when it's a business loaning the money that desire to help doesn't exist, and it becomes a business partnership instead.
You're supposed to use Mudarabah[0] preferentially, which is dramatically less of a lazy hack around usury rules than Murabaha. (Also as a English speaker, those words are very hard to distinguish.)
