Why weren't the sins of the people before the Flood charged to them?
Their sins were not charged to their account because the written Law had not yet been given. God could not impute that pre-Flood wickedness as lawbreaking, since no written Law existed for them to violate. As Scripture reasons, you cannot break a law if there is no law.
Les explains that up until the time the Law was given, God could not impute the wickedness that existed before the Flood or put it to those people's account, because they were not breaking any written Law. The Law was added later, and its purpose was to convict people of sin by showing how far mankind falls short of God's standard. Once the Law was given, the whole world became responsible and guilty before God, because now there was no excuse for things like adultery, thieving, and coveting. Before the Law, however, you cannot break a law if there is not one. Les notes that the pre-Flood people will still come before the Great White Throne and be consigned to their place of judgment, but God will not judge them in the same way by pointing to all their Law breaking, because they had no Law.