Why did ancient inscriptions use turtle backs?

In the eyes of the ancients, turtles belong to the longest-lived creatures. Since it is the longest life, it is naturally the most spiritual. Divination of turtle's back has been popular since Xia and Shang Dynasties, because there are not as many turtles as expected. So only some important events will choose this "sacred" creature. In general, animal bones are in the majority.