paul is speaking to the churches leaders that is what the epistles were for it was written to the churches. so they can teach the gentiles, this is what we have for Paul;s writings epistles, theses we specific letters and had a specific purpose.Paul is speaking of gentiles, not Jews! What do you think, the legalistic levitical laws are written in gentiles hearts but the moral law is left out?
Paul's epistles were pastoral letters written to specific churches and individuals, usually to solve specific problems. Many of those churches contained Gentile believers because Paul had a special ministry among the Gentiles, but his letters were not originally written as universal rule books detached from their historical context. They addressed real issues faced by particular congregations, while Jesus' own teachings were given as the foundation that His disciples were to teach to all nations.