Well if I were a theist I would say because Jesus Christ came and changed the rulebook adding a new covenant for Christians. I've debated this myself many times, but they ignore the words of Christ saying "Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them." The Christians claim the Old Testament law was given to the nation of Israel, not to Christians. One of the reasons was Christians were trying to persuade pagans to convert to Christianity, but many would have said I like the idea of your religion but I would rather eat a ham sandwich and some of your other laws are just too extreme. To make it easier to gather more new followers, they changed and manipulated the rulebook.