The first answer is simple: hereditary rule. For most of this period, emperors were not chosen on the basis of their ability or honesty, but simply because they were born in the right family.

Patrician families made up the ruling class, meaning they were in jobs that had a lot of influence in how Rome and the empire were ruled. The majority of the population were known as plebeians and ...

The Roman Senate started life as an advisory council, filled entirely with patricians ... Potential candidates were nominated by the emperor and the elections were merely a formality.