Why does the Lord not count sin against a blessed person?
Because their iniquities are forgiven and their sins are covered. Scripture calls this blessedness righteousness imputed without works: God credits righteousness apart from works, so the forgiven person's sin is not charged to their account. That is why the Lord does not put sin to their account.
Les explains from Romans 4:6-8 that David describes the blessedness of the man to whom God imputes righteousness without works. The blessed person is the one whose iniquities are forgiven and whose sins are covered. In verse 8, the Lord will not put sin to that person's account. The reason given is simple: their sins are all forgiven. Les connects this to the preceding analogy of striving for reward—those men pursued a perishable wreath, but believers have something eternal in the heavens—yet the foundation of blessing here is God's imputation, not human works. Because God has forgiven and covered their sin, He does not charge it against them.
Blessed is the man to whom the Lord will not impute sin.