Empires rarely fall overnight. Their destruction often takes many decades to unfold. Such was the case with the Roman Empire. Much of the decay of Rome happened from within. Economic problems were the major culprit. Overtaxation and inflation, caused by the devaluing of the coin system, resulted in a wide gap between rich and poor. The Romans also relied too heavy on slavery which exacerbated the income gap. There was also a decrease in the number of slaves available for service as Roman expansion ground to a halt. The empire was also not able to defend against piracy in the Mediterranean which seriously disrupted their trade capacity. The decline in economic power weakened Rome and left its frontier vulnerable to attack. Rome also suffered from inadequate leadership after Marcus Aurelius died in 180 AD. While there were some effective leaders after Aurelius, the trend for centuries was poor and corrupt political leadership.
At the same time that Rome was in decline, their eastern counterpart in Constantinople was growing in wealth and trade. Diocletian had divided the empire to make governance more efficient. In doing so, he actually left Rome in the west more vulnerable.