Why did God decide to do something completely different after two thousand years?
After two thousand years of utter human failure, God chose a totally different course because the whole race had rejected His simple provision: conscience-prompted confession and blood sacrifice. They rebelled, ignored His offer of salvation, and pursued their own false worship, so judgment came at the flood and again at Babel.
Les explains that God had given the whole human race an opportunity for salvation. All they had to do was be prompted by conscience that they had sinned and rebelled against the laws of conscience God had placed within them; then they were to bring a blood sacrifice and confess their sin, and God would accept them. It was that simple. But they rejected it totally. It got so bad that God destroyed the generations with the flood and started over, in so many words, with Noah and his three sons. Yet within two hundred years the offspring of those three sons were again in total rebellion and total ignorance of God's offer of salvation, trying to make their own way and approach heaven through their own false worship, resulting in the tower of Babel. Therefore, after two thousand years of utter failure on the part of the human race, God said He was going to do something totally different.