Because of the righteouness of Abraham

Why was Isaac blessed?

English Standard Version Chapter 26

And Isaac went to Gerar to Abimelech king of the Philistines. 2 And the LORD appeared to him and said, “Do not go down to Egypt; dwell in the land of which I shall tell you. 3 Sojourn in this land, and I will be with you and will bless you, for to you and to your offspring I will give all these lands, and I will establish the oath that I swore to Abraham your father. 4 I will multiply your offspring as the stars of heaven and will give to your offspring all these lands. And in your offspring all the nations of the earth shall be blessed, 5 because Abraham obeyed my voice and kept my charge, my commandments, my statutes, and my laws.”

God was going to be with Isaac and was going to bless him and his offspring not because of his own righteousness but because of Abrahams.