Answer: Rome fell because it had conquered too much territory.
In this excerpt, Gibbon is arguing that the main reason why the Roman Empire fell was because of the difficulty of maintaining stability over such a large territory. He claims that "the causes of destruction multiplied with the extent of conquest," arguing that the more territory they acquired, the more potential for problems they found. He thinks this is a natural and inevitable end for empires that reach the "excessive greatness" that Rome enjoyed.