Why haven't Les and his listeners gone down the same track into idolatry and immorality?
Only by the grace of God. The same downward track of idolatry and immorality that empires, nations, and individuals follow could have overtaken them. Apart from that grace, they too could have succumbed to idols and ended in immorality, but God's grace prevented it.
Les explains that the downward track has been underway especially since the Tower of Babel, and it claims empires, nations, and individuals alike. He warns listeners never to lose sight of that pattern. Then he says that but for the grace of God, you and I could just as well have taken that same track and succumbed to idols, ending up in immorality. So the only stated reason they have not gone down that path is God's grace.