Because Chinese or Iranian imperial succession was defined much more clearly. In China for instance you actually had to be descendant from the previous emperor (through a concubine is fine and common, but most of the time you had to be descendant from the imperial line) which greatly increased the predictability of succession and stability, as well as limiting the pool of potential candidates for inheritance.

The problem with the Roman system was that the Augustus was not suppose to be monarch (at least not until Diocletian anyway), and at least initially the emperor maintained the trappings of Republican institutions. The net result of this is that succession gets to be a lot more ad-hoc and dependent on the political climate of the time. If you are not a monarch it gets a lot harder to justify why there should be an orderly succession from you to your eldest son to your younger sons etc. Because it's not a monarchy there is far less need to have imperial blood in your veins to take the purple, which means that it get s a lot easier for popular general X to proclaim himself emperor (see the crisis of the third century). All of which means succession is much less stable and much less predictable.