[qwytw]

The Roman period was followed by general population contraction and a total collapse of urban centers. The city for Rome itself did not reach the population it had in the 200's AD until the early 20th century. Of most ancient Rome was an abandoned or sparsely inhabited ruin (much like many other Roman cities) which is why so many buildings survived.

If Rome had remained a densely inhabited city most of the ruins would had likely been completely destroyed and built over (besides a few temples which were converted to churches). If we look at smaller Roman cities like Cadiz, London, Paris, Milan etc. there aren't that many traces of Roman times there because their population levels eventually rebounded during middle ages.