Something else no one has mentioned yet is that in ancient
times the cities were wholly parasitic. That is, not only did they
consume lots of resources, but they produced *nothing* in
return, so they were a huge drain on the economy. Not the
only factor, of course...
BTW, on a history of the Western World telecourse, the prof
said that maybe the question that people should ask is not "why
did Rome fall", but "How did Rome last so long?" After all,
515 BC -> 1453 AD outlasts everything except Ancient Egypt.