Romans were terrible at maintaining dynasties
The Persians (repeatedly), Ottomans, Egyptians, Chinese and many other kingdoms large and small have managed hereditary or familial dynasties for over 100-500 years at a stretch. Perhaps the Mongol and Turkic empires aside, in many of the enduring dynasties these kings are not often killed in battle or assassinated; and when they are it's by a brother, in law or cousin.
Romans are all over the place family-wise. In the late Empire family is almost irrelevant. Over a third of the Emperors murdered, almost as many battle deaths or enemy executions, six known suicides. These statististatistics show an insanely unstable monarchy, they may have done well imperially but dynastically they compare so poorly as to be comical.
The fact is that the Roman Empire never was a dynastic monarchy, at least not constitutionally. The Principate was a bizarre and unstable equilibrium between a formal Republic and a de facto military dictatorship.
The form of the state remained largely untouched in the transition between the Republic to the early Imperial Age. Contrary to Palpatine in Star Wars , Roman emperors never suppressed the Senate, most central Roman magistrates of the Republic continued formally untouched, including the consulate, even if they were largely devoid of real power. And that was at the center, in Rome. At a local level municipia in Italy and in other territories with Roman citizenship continued to be governed by elected officials (election campaign graffiti in early imperial Pompeii are a clear example!). Roman emperors never took the title of 'rex' (king), which was hateful to Romans. They used titles such 'augustus' (the beloved), 'princeps' (first citizen), 'imperator' (general, military commander) and took the name of 'Caesar' (who himself refused to be king, taking the title of 'dictator for life').
To my knowledge there were no written family based succession laws in the Roman empire. Exactly because the state was formally a republic! Emperors could indicate a successor but things went smoothly only when this choice was given legitimacy by the Senate AND military backing from the Legions and from Rome's Pretorian Cohort - which happened, well, rarely...
The only period in which succession sort of worked was the age of the Antonine 'adoption emperors': Nerva -> Trajan -> Hadrian -> Antoninus Pius -> Marcus Aurelius, where each emperor adopted as son his appointed successor, who usually was a relative. Marcus Aurelius had adopted Lucius Verus and made him co-emperor but here things went wrong because Verus got the plague and died. The throne passed to Commodus, Marcus Aurelius' biological son and the golden adoption age was over... It is in fact surprising that it did not go wrong earlier, because the adoptions were the result of complex negotiations, brokered by the Senate...
Then Late Empire was entirely a different beast, but surely not a dynastic monarchy in any traditional sense. The military element was prevalent. Diocletian tried to put order in the succession mess with the Tetrarchy system, which was a bizarre chain of succession between itinerant regional commanders and vice-commanders with their travelling état-major and court. It never really worked.
I don't know how the Persians managed to maintain family dynasties, since they had no concept of peaceful transfer of power. As soon as the current King of Kings died, it was civil war no matter how legitimate the named successor was. Every uncle, cousin, nephew, brother, and step-whatever jumped into the fray to seize the crown -- and the carnage didn't end until there was only one possible heir left. When Persian subjects said of the king "may he live forever" they really meant it!
Because the relatively independent satrapies that developed were harder to subjegate individually than en masse; though Alexander was able to exploit the weakness of a dynastic struggle. As to why the dynasties lasted, it's because they operated on a practical variant of the Law and customs of most steppe tribes, especially when it came ro the aristocracy and nobles. It would be a boldor reckless thing to try to replace a chief due to complex kin and commercial issues; and it's mainly foreignish (though basically the same background as the Persians, Hyrcanians and inhabitants of lower Aria) still wild tribes staged the coups, such as the Parthian nominal subject tribe or the provincial administrator and chief who replaced the Parthians afterward. It's more like the Holy Roman Empire combined with a tradition of steppe horse lords clans than Byzantine or English politics.
Originally Posted by seneschal