Because humans are not good by nature. (Psa. 14:3-5; Rom 3:10-18) ). Men by their internal make up are at "enmity against God and are not subject to the Law of God" (Rom. 8:7) Only the Gospel of Jesus Christ works to give life to creatures that are "dead in trespasses and sin" and only the supernatural working of the Gospel through the Holy Spirit can supply the power to "obey" Him (Heb. 5:9). Therefore regardless of what "moral law" any society imposes it's futile- simply because they can not and do not want to obey the law of God, or moral laws, or civil laws, etc.. That is why Ezekiel 11:19 points out that God will one day give His people a new heart and take out the stony heart of stone and replace it with a "fleshy" heart that wants to do what God says is righteousness rather than some strange "moral" Islamic law imposed on people with fear and torture. Since the divine intervention of God is real, you don't have to threaten humans with Islamic laws or any other laws to force people to do what is good and moral because when they are "born again" it becomes part of their Christlike nature, to "do the will of Him" that made us. And this happens without the threat of jail.
This video shows a prime reason why the law can't save you and why God does not grant salvation by being good. How come?