Look, I hope you aren't expecting a six-volume masterpiece….

Davis summarizes the reasons for the fall (actually a slow, grinding, gradual decline) of Rome into four:

…by the beginning of the fifth century the Roman Empire had been suffering from economic decline for at least two centuries.

…the Romans were already being "barbarized."

…the barbarians were already becoming Romanized

…the precise manner in which the barbarian settlements were made.

I will expand on one of these: the economic decline.  Let's just say it is expensive to run an empire.

The primary cause of the economic decline was that the Roman economy had become unproductive.  Wealth was gathered via conquest; the people ate free corn from Africa and Sicily, selling nothing in return.  The chief trade was money lending.  When the frontier of empire could not be further extended, growth in this manner stalled – the unproductive Romans ran out of productive to consume.