In such a large empire that lasted so long, there is no one answer that can be given. All of the people above pointed out different reasons of why the empire came to an end. And in my opinion all of them are good answers. However, I'd like to point out one new perspective. The eastern empire, that is, Constantinople did not fall. It lasted for much longer until the Ottoman Turks in the 15th century. And even during the 5th century much of the resources of the empire was sent to the east. The west had the glorious past reputation, and historians, like Ammianus, called Rome, the eternal city, but in reality, the real seat of the empire was in Constantinople. Therefore, only to point out the fall of Rome as the fall of the empire is to be too western in one's thinking. That said, if I had to take a guess on why the empire fell, it is due to military issues. Too many foes with too little resources spells the end.