There are many theories about the ultimate fall of the Roman Empire in 476 AD. In truth there is no simple answer but many interwoven reasons based on ongoing events over many decades. Simply put - nothing worked anymore. To fix one problem, would mean to create others. It was time for change and everyone within - or connected to the Roman Empire - felt it coming.
The downhill slide began with the Crisis of the Third Century. Then after nearly half a millennium of rule - and split into two Empires - Western (Italy) and Eastern (Constantinople) - the Romans finally lost their grip on Europe in the 400's. This century left in its wake untold devastation, political chaos and some of the most fascinating and problematical issues in history.
Many would blame the usual corruption one finds in political, economic, religious, and social systems even today. The edifice of the Roman Empire had been built on unsound foundations to begin with - its fall in the final analysis - inevitable. It would seem the architecture blueprint of any reality is designed to serve a period of time in the human experience, then decline and end. The human experience is nothing more than a collective consciousness hologram set in physical reality to study emotions. That too is in the process of change.
The underlying causes for the fall of the Roman Empire are no different than the way the world works, or doesn't work, today:
Political Corruption
Economy, Inflation, and Unemployment
Excessive Military Spending with ongoing Wars
Decline in Morals and Values
Public Health
Housing
Social unrest and the need for personal freedom
Church vs State