Certain questions are perennial, and one of those is why the Roman Empire fell. Peter Heather provides an explanation that is detailed and nuanced, and surely better than Edward Gibbon’s work. First of all, it was only the Western part that fell at the time he is writing about (5th century, basically), and there were certain accidents of fate that helped the outcome. At the same time, the Eastern part of the Empire remained strong and vibrant at least up until the rise of Islam in the 7th century, and technically the Easter empire did not fall until 1453 when the Ottoman Turks finally conquered Constantinople.
The fall of the western Empire was a complex phenomenon made possible primarily by changes in the political organization of the Germanic tribes (mostly Gothic) brought about by two different forces. One was the move of the Huns from the area to the east, who moved into the Hungarian plain in the 4th century and began absorbing Germanic groups into their own empire. And the other was the pressure from Rome itself. By its commerce, its military pressure, and its diplomacy, Rome essentially molded the Germanic tribes into larger and more effective political units that could seize the opportunities they were given as a result of military reverses Rome faced. And because the elite groups in the Empire held most of their wealth in the form of landed estates, once the balance of power began to shift they looked to transfer their allegiance to the new barbarian kingdoms. You can’t pick up land and take it with you, after all.
All in all, a very good book on the subject