It is not that God failed to keep his promise to them. But only some of the people of Israel are truly God's people,
and only some of Abraham'sn descendants are true children of Abraham. But God said to Abraham: "The descendants I promised you will be from Isaac."
This means that not all of Abraham's descendants are God's true children. Abraham's true children are those who become God's children because of the promise God made to Abraham.
God's promise to Abraham was this: "At the right time I will return, and Sarah will have a son."
And that is not all. Rebekah's sons had the same father, our father Isaac.
But before the two boys were born, God told Rebekah, "The older will serve the younger." This was before the boys had done anything good or bad. God said this so that the one chosen would be chosen because of God's own plan. He was chosen because he was the one God wanted to call, not because of anything he did.
As the Scripture says, "I loved Jacob, but I hated Esau."
So what should we say about this? Is God unfair? In no way.
God said to Moses, "I will show kindness to anyone to whom I want to show kindness, and I will show mercy to anyone to whom I want to show mercy."
So God will choose the one to whom he decides to show mercy; his choice does not depend on what people want or try to do.