Stone buildings actually take a surprising amount of maintenance to keep from falling apart. Stone can be worn down by rain and weather, foundations shift and settle, and lightning strikes and open-flame lamps can cause fires. York Minster, a medieval cathedral I visited last summer, has suffered from all of these things, and there are full-time stone masons constantly replacing damaged and aging sections to keep it from collapsing from normal wear-and-tear.

In antiquity, maintenance was a constant concern. During the most properous periods of the Roman empire, elite patrons would pay vast amounts of their incomes to refurbish, remodel, and replace aging monumental urban architecture. But by the fourth century, several things were changing which began to discourage this reinvestment. New laws made it increasingly expensive to hold public office within cities, and many elite families moved out of the city and invested their resources in rural villas (semi-private/public country palaces). At the same time, the church was becoming an increasingly attractive recipient of patronage, and other elites began donating money once used to maintain civic buildings to bishops (who used it to feed the poor, build churches, and establish their own country estates). As these changes were happening, the centers of power in the western Roman empire were shifting away from the city of Rome toward Milan, Ravena, and other northern-Italian cities (and, of course, to Constantinople). As these cities grew, money that had been used to maintain Rome's monumental architecture was diverted elsewhere. The destruction of Rome in 410 didn't help matters, but what really did it in were the factors above, each of which diverted resources away from completely rebuilding, renewing, and maintaining the city in its previous monumental glory.

As resources were diverted and patrons spent their money building and maintaining structures in other parts of the Mediterranean world, people in Rome began pulling down decrepit buildings to recycle their materials. Roman stone buildings were held together with iron staples; there was a lot of useful material in a run-down badly maintained building beyond the stone (which was, itself, incredibly valuable). And many buildings appear to have been replaced with parkland - Rome was full of chestnut trees by the early middle ages. It's likely that many of the people doing this remodelling preferred the new parkland to the crumbling, dangerous old buildings it replaced. Compare this with the current situation in Detroit, MI, and you can get a good idea of many of the social processes involved.