Why would you have slavery back and no separation of the church and state? If the Christian Church can recognise it needed a New Testament, then why not a new

Why would you have slavery back and no separation of the church and state? If the Christian Church can recognise it needed a New Testament, then why not a new US constitution. I'm sure many of religious followers would accept the need for change and updates, as technology and business in general demonstrates on a daily basis. Or is the resistance to change simply demonstrating peoples needs or desires to live in some rose tinted past of human history, that has white washed the abominations of the past? If it does, should these people be making decisions for lots of people?