Why did God turn to the Gentiles with the age of grace?
God turned to the Gentiles with the age of grace because Israel rejected the offer of the kingdom in unbelief. The Old Testament prophecies and Peter's preaching in Acts presented Jesus as the Christ and King, but the nation refused to believe, so God shifted His program to the Gentiles.
Les explains that according to the Old Testament, the ascended Christ would not have stayed long at the right hand until He returned to set up His kingdom, if Israel had obeyed and believed Jesus was the Christ. This was the whole concept of Peter and the eleven preaching to Israel in Acts: "The one you crucified was the Christ... He is our King... He can yet come and be our King." The kingdom promises were prophesied to fall in place, and Israel could have evangelized the Gentiles. But in reality, Israel rejected it in unbelief, and God instead turned to the Gentiles with the age of grace.