Jamieson, Fausset and Brown on Deuteronomy 23:19-20

19-20. Thou shalt not lend upon usury to thy brother ... Unto a stranger thou mayest lend upon usury--The Israelites lived in a simple state of society, and hence they were encouraged to lend to each other in a friendly way without any hope of gain. But the case was different with foreigners, who, engaged in trade and commerce, borrowed to enlarge their capital, and might reasonably be expected to pay interest on their loans. Besides, the distinction was admirably conducive to keeping the Israelites separate from the rest of the world.