Because many Christians are not being consistent. That's the point of my article. You can't say Christianity has moral absolutes when you also say God uses different moral principles in different ages (OT and NT). Also, if you insist that anythign God does is moral, then there is NO objective moral principle (something that exist independently), only subjective moral dictatorship.

Moral persuasion depends on whether both parties agree to some fundamental principles. If both parties agree, then they can dialogue and challenge each other. It does not matter whether those principles are objective or subjective. They can still say, "You should do or agree with xyz because of our agreed fundamental moral values." One of those principles is "it's good to be consistent."

You can argue that people don't have to be consistent if you want to. I would not argue with you on this then.