Why was it important that Isaac's wife be from within Abraham's family?

In Genesis 24, Abraham tells his servant to go back to his homeland and find a wife for Isaac from Abraham's country and Abraham's relatives. First of all, I suppose I can understand why Isaac was not allowed to marry a Canaanite. Abraham didn't want Isaac, the son of promise, to be led astray by a wife outside the covenant, right? But that still doesn't explain why the wife had to come from within Abraham's relatives. Sure, the servant could go back to his hometown; but surely there were other eligible young women among other families?