Lack of orderly imperial succession
"Finally, among the political causes of the fall of Rome must be named the lack of a rule or principle of succession to the throne. The imperial crown, during the five centuries with which we have had to do, never became hereditary or regularly elective. Almost from first to last, as we have seen, the emperor generally reached the throne by irregular and violent means. The strength of the empire was wasted in constantly recurring wars of succession. Could a dynasty have been established in the first century, and had there grown up among the people a feeling of loyalty towards the imperial family, like that, for instance, of the Scotch to the House of Stuart, this sentiment would have given security and stability to the throne, and the history of the empire might have been wholly different from what it was. 9 History of Civilization in Europe, Lee. II."
Rome, Its Rise and Fall By Philip Van Ness Myers 1901