Why do appendicitis pains start in navel?

It's embryology: The appendix is part of the embryonic midgut. It's blood supply comes from the superior mesenteric artery. The nerves that supply the bowel run along with the blood vessels. With that in mind, the midgut is sensed in the midabdomen - ie periumbilical. The other two parts are the foregut and hindgut. The foregut is felt below the breast bone and the hindgut is felt above the pubic bone.