On the topic of the fall of Rome, Edward Gibbon wrote that it was more surprising that it lasted as long as it did than that it fell, but if we want to know the reason, it was because Rome had grown too powerful and too big and couldn't hold up under this weight. Here are his words:

"The decline of Rome was the natural and inevitable effect of immoderate greatness. Prosperity ripened the principle of decay; the cause of the destruction multiplied with the extent of conquest; and, as soon as time or accident and removed the artificial supports, the stupendous fabric yielded to the pressure of its own weight. The story of the ruin is simple and obvious: and instead of inquiring why the Roman Empire was destroyed we should rather be surprised that it has subsisted for so long."

Source: The Decline and Fall of Rome