Barbarians took over
Because it was too big to controll ;)
Decline in Morals and Values
Even during PaxRomana (A long period from Augstus to Marcus Aurelius when the Roman empire was stable and relativly peaceful) there were 32,000 prostitutes in Rome. Emperors like Caligula and Nero became infamous for wasting money on lavish parties where guests drank and ate until they became sick. The most popular amusement was watching the gladiatorial combats in the Colosseum.