Hubris.
It's the death of all empires.
Also - and it may be co-incidence - there was major climate shift to the cooler as the Dark Ages began.
Christanity is a tempting culprit but looking to the duration of the east ultimately unconvincing. Ditto the plunder economy.
Both doubtless were contributing factors.
I think,simply, that Rome rested upon her laurels. A new God perhaps let her forget the capricious nature of deities
