Restriction to the land
"William Carroll Bark argues in Origins of the Medieval World (1958) that the Empire fell due to the efforts of keeping it together. The roots of feudalism developed when the colonus (a pre-cursor of the serf) was legally bound to his role of tenant farming so that collection of taxes would be easier. The Imperial government collected fixed grain taxes from tenant farmers. Since it was impossible for the government to keep track of its large grain supply, the middle class was legally required to collect taxes. With citizens bound to certain local roles, feudalism began developing before the Middle Ages had even begun."
"Bark also cites the scarcity of gold in the late empire as a reason for its decline. Currency inflated as it was no longer made of real gold. No longer wanting to be paid with money, soldiers and wealthy citizens chose instead to be paid with actual objects of value. It became difficult for the government to retain any money for itself, and it began to resort more and more to cheaper mercenaries to defend it."