Why did Abraham and Isaac instruct their children to marry only from their clan?

Abraham was from a pagan family, separated by his call to God. His families I supposed weren't different from the Canaanites in pagan worship. In some instances, the Canaanites proved more morally upright than Abraham and Isaac by instructing their fellows not to interfere with Isaac and Abraham's wives (family), meting with dire consequences anyone who does break such binding law. Genesis 26: 6-11 If this proves true, why did Abraham and Isaac instructed the heirs of the covenant never to marry from the Canaanites (pagans) but should marry from their families (pagans) in Padan-aran?