Why does Galatians keep going back to the Old Testament and to the Jew?
Galatians returns to the Old Testament and the Jew because Paul is contrasting Law and Grace, using Abraham and the allegory of Isaac and Ishmael as pictures of both. He appeals to Israel to leave legalism and blindness and enter the Light of God's Grace.
Les teaches that Galatians is built on the theme that you are not under Law but under Grace, and that theme runs through all six chapters. Paul constantly referred to Abraham, and Isaac and Ishmael were used as pictures of Law and Grace. So Galatians keeps flipping back into the Old Testament, using the Jew as examples and appealing to the Jew to come out of blindness and legalism and step into the Light of God's Grace. This covered the period while Paul was roaming the Roman Empire and establishing churches, before the Acts 28 interruption, his arrest, and his imprisonment in Rome.