There are several issues involved in the fall of the Western and and the continuation of the Eastern Roman Empire. They all play a part to varying degrees at different times.
By the time of Adrianople, the physical city of Rome had lost its importance in the empire. The idea of Rome was what was important. Constantinople was now the most important city of the Empire.
The Georgraphy of Europe and Asia Minor played role in the fall of the Western Empires as well. The lines of defense in the west were long, and sparsely defended. According to previous posts, the legion sizes were now smaller. The importance of Constantinople in relation to Rome played a major role in the placement of the bulk of the army . The physical governmental offices and staff were primarily located there as well. Caucasus Mountians created a physical barrier and the Parthians a buffer zone.
The actual physical movements of various tribes either in an aggressive mode or on the run from other tribes behind them also played a role. The Huns approached from the east. Looking at the map of Europe, there was more room for the Barbarians (Goths, et al) to move west then south than directly south from the Hungarian plane. The push in Europe seems to have been east to west by successive tribes.
One of many major faults of the government of Rome was corruption. Officials, military personal often pursued their own ends to the detriment of the empire and welfare of the people of the empire. The treaties with 'barbarians' were not honoured.
This is just a few of the ideas swirling my head. Unlike others, I read, I swirl, and the ideas form. I apologize for not having concise references for my ideas.
I have read Ward-Perkyns, his is a wonderful antedote to the current attitude of benevolent peaceful incursion of 'Barbarians' into the Western empire,. Also Goldsworthy, parts of Peter Heather, Alesandro Barbero's book, and others.