Why couldn't God charge the wickedness of the people before the Flood to their account?
Because no written Law had yet been given, God could not impute the wickedness of the pre-Flood people to their account; you cannot break a law that does not exist, so there was no transgression to charge against them. The written Law came later.
Les teaches that up until the time the Law was given, God could not even impute all the wickedness before the Flood or put it to those people's account, because they were not breaking any written Law. He explains that you cannot break a law if there is not one. The Law was given for one purpose: to show mankind how far they fell short of God's standard, to stop every mouth, and to make all the world guilty before God. Even though the pre-Flood people will come before the Great White Throne and be consigned to their place of judgment, God cannot say, look at all the Law-breaking you did, because they had no Law. But as soon as the Law was given, the whole world became responsible and without excuse for adultery, thieving, coveting, and all the way down through the Ten Commandments.