Because the Jews didn't keep their part of the Covenant God made with the Jews, God has made a New Covenant which can involve all nations - not just the Jews.
God’s New Covenant has enabled people to have theie sins forgiven:
27 This is my covenant with them,
when I will take away their sins.”
The New Covenant is based on faith in Jesus’ sacrifice whereby our sins can be forgiven. At the Last Supper, Jesus said:
28 for this is my blood of the new covenant, which is poured out for many for the remission of sins.
The New covenant which promises an "eternal inheritance" was confirmed by God raising Jesus from the dead:
15 For this reason he is the mediator of a new covenant, since a death has occurred for the redemption of the transgressions that were under the first covenant, that those who have been called may receive the promise of the eternal inheritance.
20 Now may the God of peace, who brought again from the dead the great shepherd of the sheep with the blood of an eternal covenant, our Lord Jesus, 21 make you complete in every good work to do his will, working in you that which is well pleasing in his sight, through Jesus Christ, to whom be the glory forever and ever. Amen.
So ultimately, the Covenant which God made to Abraham is extended to all people who put their trust in the redeeming sacrifice of the Lord Jesus:
25 You are the children of the prophets, and of the covenant which God made with our fathers, saying to Abraham, ‘All the families of the earth will be blessed through your offspring.’
29 If you are Christ’s, then you are Abraham’s offspring and heirs according to promise.