There are many theories as to why Rome fell, but none of these have one chief explanation. I believe public health, barbarian invasions, military spending, inflation, and religion were the most important factors that caused the Roman empire to plummet.
The Roman Empire had numerous environmental and public health issues. The lead utensils and pipes caused lead poisoning of the aristocracy. This was the root of the lowering birth-rate and intelligence level of the empire. Plagues and sickness also made a decline in population. Disease was commonly spread at the Colosseum where blood and death was present.
The army of the Roman empire held the German barbarians back for decades. To fight the civil war in Italy,