Because (διοτ, again, δια, οτ).

By the works of the law (εξ εργων νομου). "Out of works of law." Mosaic law and any law as the source of being set right with God. Paul quotes Psalm 43:2 as he did in Galatians 2:16 to prove his point.

The knowledge of sin (επιγνωσις αμαρτιας). The effect of law universally is rebellion to it (1. Corinthians 15:56). Paul has shown this carefully in Galatians 3:19-22. Cf. Hebrews 10:3. He has now proven the guilt of both Gentile and Jew.