Sirach 44:23 "The blessing of all men and the covenant he made to rest upon the head of Jacob; he acknowledged him with his blessings, and gave him his inheritance; he determined his portions, and distributed them among twelve tribes." 

The covenant rested on the head of Jacob, after Abraham, and then Isaac, but note that the blessing is "of all men". God chooses and calls those who will help Him in bringing this blessing to fruition. He called Abraham, and began the covenant with him, but not so that only Abraham and his descendants would be blessed, but so that eventually the blessing would go out to all men. Isaac and Jacob, in turn, continued the covenant, and were blessed by God. From Jacob, the covenant went out to the twelve tribes, the whole Jewish people. Eventually, God Himself came as one of those Jewish people and completed the passing of the blessing and the covenant to all men, just as He promised.

God's plan is eternal, so it seems to us to take a very long time to play out. But God always keeps His promises, including the one that began with His covenant with Abraham, the promise to bless all of humanity.