Why did God set aside the nation of Israel according to this teaching?
God set Israel aside after the nation had spent seven years constantly rejecting Peter and the eleven and their appeal. At that point, God gave up on Israel as a nation and turned to the Gentiles, saving Saul by grace and sending him to them.
Les teaches that Saul was on his way into Gentile territory toward Damascus to arrest believing Jews, but God in grace saved him and immediately informed him he would be sent to the Gentiles. That is the first indication in Acts that God would bring Gentiles into the picture. Though the Old Testament showed Israel was to be a light to the Gentiles and Israel's Messiah was to be that light, it was only through Israel. Since Israel had seven years of constantly rejecting Peter and the eleven and their appeal, God gave up on the nation and set them aside. Soon afterward Roman armies destroyed the temple and Jerusalem, and the Jewish people were dispersed until modern times; they returned to the land, became a sovereign state in 1948, and prophecy moves toward fulfillment. Les emphasizes no prophecy is directed to the church.