There are many reasons why Rome fell:
1. Invasions (plural) by Barbaric Tribes (there were many).
2. Economic Instabilities.
3. Ironically, the rise of the East over the West (it lasted wayyy longer than the West).
4. Overexpansion and Overspending on the Military.
5. Government Corruption (as if that's a switch, Rome's Senate was bad).
6. The Huns (oh boy).
7. Christianity and the changes it brought to the Empire.
8. Weakening of the legions by recruits outside the citizenry (previously only Romans could be legionaries but that changed and they were of a lesser calibre because of it).
It has been my pleasure and duty to serve.