Why were God's covenant promises given only to Israel?

Les teaches from Ephesians 2:12 that Gentiles were aliens from the commonwealth of Israel and strangers from the covenants of promise, having no hope and without God in the world. Israel alone held those covenants; Gentiles could not claim them as their own. God was dealing with covenant promises exclusively for the nation of Israel until she became a prepared, spiritual, obedient people. Then He would send them into the Gentile world to bring Gentiles to a knowledge of their God. But Israel, after 2,000 years of being told they were special, became prideful and could not accept that anyone else was worth anything.