The Eastern Roman Empire, or Byzantium, managed to survive much longer then the Western Roman Empire because of multiple factors.
Some of the most important factors are that Byzantium had a strong leadership. The hierarchy was well placed, and the policies of the empire were good and prosperous.
Also, Byzantium was very wealthy, controlled important trade roots, had very well developed agriculture, and produced lot of goods, thus making the empire economically strong.
The military was well payed, well equipped, well organized, and it was loyal to the crown.
