We are used to things starting out small and simple and then progressing--evolving--to become ever more complex and sophisticated, so this is naturally what we expect to find on archaeological sites. It upsets our carefully structured ideas of how civilizations should behave, how they should mature and develop, when we are confronted by a case like Gobekli Tepe that starts out perfect at the beginning and then slowly devolves until it is just a pale shadow of its former self.