Han emperors were so successful at maintaining power because:

The Silk Road established steady trade.
Peasants were regularly pacified and happy.
Succesive emperors proved to be strong leaders.
They oversaw important inventions of paper, wheelbarrows and the like.

I don't know if you still need this answer, but i think it is Successive emperors proved to be strong leaders. Please correct me if im wrong. Hope this helps!

Explanation:

the hyksos invasion. the hyksos were able to attack egypt successfully because of their skills in using bronze weapons, composite bows and the use of the horse and chariot during the invasion. although the egyptians depict a barbarian-type overthrow, the truth is the seizure was likely a relatively peaceful one.