Why did God's choice of Jacob over Esau surprise Isaac and Rebekah?
Isaac and Rebekah were surprised because Middle Eastern custom made the firstborn twin the rightful heir. Even among twins, the elder son received the inheritance. The revelation that the elder would serve the younger, and that Jacob would be ascendant and carry the promises, reversed their customary expectation and human reason.
Les explains that verse 11 speaks of Jacob and Esau while still in Rebekah’s womb, neither having done any good or evil, so that the purpose of God according to election might stand, not of works but of Him that calleth. The words “the elder shall serve the younger” were contrary to Middle Eastern custom, where the firstborn twin was the eldest son and rightful heir of the inheritance. This flew in the face of what Isaac and Rebekah understood: the younger, second twin, Jacob, would be the ascendant one and the one through whom the promises would come. Les says this is against human reason, and he views it in light of God’s foreknowledge—seeing the end from the beginning, God knew Esau would be destitute of faith.