One of the major things going against the survival of the Western empire was the fact that the local landowners in the Senate and the vast apparatus of the Catholic Church had more or less turned their back on the empire and disassociated themselves from it. In doing so, they sapped the strength of the army and of the administration, largely because they came to a point where they realized that they could survive without it.
Another problem that distinguishes the Western Empire from, say, China or the Byzantines is that it was unwilling and unable to assimilate the barbarians. Peter Brown writes:
The society of the western provinces of the Roman empire was fragmented. In the late fourth century, boundaries had hardened, and a heightened sense of identity had led to harsher intolerance of the outsider. Senators who had participated in an impressive revival of high standards of Latin literature were little inclined to tolerate a 'barbarian.' ... As a result, the barbarian tribes entered a society that was not strong enough to hold them at bay, but not flexible enough to 'lead their conquerers captive' by absorbing them into Roman life.
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The barbarian settlers in the West found themselves both powerful and unabsorbable. They were encapsulated by a wall of dumb hatred. They could not have been 'detribalized' even if they had wanted to be, because as 'barbarians' and heretics they were marked men. The intolerance that greeted the barbarian immigration, therefore, led directly to the formation of the barbarian kingdoms. To be tacitly disliked by 98 per cent of one's fellow men is no mean stimulus to preserving one's identity as a ruling class. The Vandals in Africa from 428 to 533, the Ostrogoths in Italy from 496 to 554, the Visigoths in Toulouse from 418 and later in Spain, up to their conversion to Catholicism in 489, ruled effectively as heretical kingdoms precisely because they were well hated. They had to remain a tight-knit warrior caste, held at arm's length by their subjects. Not surprisingly, the word for 'executioner' is the only direct legacy of two and a half centuries of Visigothic rule to the language of Spain.