Many factors working in tandem brought down the west, but the two most basic are foreign invasions/migrations and the political and administrative turmoil which made it impossible to deal with those invasions. The military had shifted, in the west, from a national army to its own institution with unique traditions, goals (namely personal profit), perspectives, and even a specific dialect of Latin pretty far removed from its civilian equivalent. This was, in part, a consequence of the increasing "Barbarization" of the military, as Germanic soldiers made up a higher and higher percentage of both the common soldiery and the officer corps; that last bit also contributing greatly to the aforementioned political turmoil as Germanic general-regents dominated court in the 5th century.
The fall of the western Roman empire is essentially the story of the imperial government's failure to accommodate migrations, extract adequate resources from the increasingly disconnected provinces, control its increasingly estranged armies, or even remain functional, let alone stable, at a court level; all this against the backdrop of Huns, (largely quasi-Romanized) Germanic invasions, and civil war.
Destabilization of central government. As Rome's power and wealth grew, there was less concern about maintaining the army, roads, and borders, and more about personal advancement which gave way to assassination and bribery as the regular means of moving up the political ladder, solely for the purpose of amassing personal wealth and power.
Also, Rome was too large. Even for an empire that size, the means to traverse it, and the armies to defend it, it could not maintain that size without proper support from the government. With an unstable central government, the money and means to support that army was weakened. As a result, foreign enemies with stronger armies (ie. the Sassanid Empire) were able to threaten and weaken Rome's dominance.
One of the other problems is imperial succession. Any son of a current emperor or rebel emperor, ambitious general, senator, governor in some far flung province, bribe the right barbarian or praetorian guard, or army coup* could in theory become emperor. Such a system leads to factionalism and the establishment of feudal enclaves.
* Army coup I am referring to one of the more obscure rebel emperors - early the morning the general was in his tent with his live-in mistress playing hide-the-sausage, when some of his men marched in. (The mistress was not too pleased about being interrupted.) They complained - we are too far away from the capital, we have not been paid in six months, and we want to make our native live-in mistresses our wives. The general replied, "There was only one problem, I am loyal to the emperor." The captains replied, "You are our emperor now, if you refuse, we will cut your balls off and move onto a commander that is one rank lower than you." He took one look at his not too pleased mistress, "Then I am your emperor." They took him out (still sky-clad) lifted him up onto their shields and made him emperor. Then the rebel emperor took his army towards Rome to make sure of his election had the force of law - he was defeated and killed. Thus, the empire died a little more.
the barbarians didn't want roman coins they were so debased that they were worth nothing , however they were keen on getting the tittle and prerogative of Duxes
it allowed them to levy taxes in kind and feed their followers , the locals were rather satisfied with this they had local produces while Roman taxation had been grievous.
as the barbarians spread through Gaul , Hispania and most importantly the bread basket of north Africa
Rome found itself in the position of Vienna post WW1 an ex empire with no tax base , no res sources and no manpower .
It kept on out of ceremonial inertia but really was a dead man walking ,
when Odoacer refused the tittle of Caesar but took the tittle of king of Italy , even this paltry pretense was dead , a few vague Illyrian claim to the contrary
the Eastern Empire was not Roman , only the Greek daughter of Rome ,
its history is wholly separate in spite of pronouncement to the contrary