See title for question. It seems to me that it was the political system. In order to keep the Empire steady it seems to me that the ambitious men needed to be given an avenue in order achieve some of their ambitions, through a different avenue than rebel and become Emperor. The way I see it, the Republic was built on the shoulders of great men and the atmosphere of Rome seemed to produce more great men than any other political system before or since. It was a system that made men need to put the state's needs first in order to get power. However with the Imperial system great men were very and far between. When Rome needed great men most in the past, the political atmosphere made one appear, no matter how many other men it broke in the process. However during the late Roman Empire there were too many rebellions and civil wars to protect Rome from the barbarians that the emperors had angered after reneging on deals that they had made.
I believe that if the Roman Imperial political system had been something like constitutional monarchy, with a Senate of 200 members and two consuls like in the Republic but with Emperor, it would have lasted much longer. The Senate and consuls could have equal power as the emperor in matters such as governor postings and (like the British government under Edward the III) money needed in order to wage war or to go on campaigns while the Emperor would be head of military and would wield much influence.