futuraprime t1_je4wdwp wrote

These two examples have pretty specific causes. u/bastardlyann mentioned the Parthenon’s being blown up in the Morean War.

The Colosseum’s a bit different. It had little purpose after the fall of the Empire—it could have comforably held medieval Rome’s entire population twice over. As a building robbed of its purpose, it not only was too much to maintain, but it was a great source of materials. Many of the iron clamps that had held its stones together were pried out and melted down. The marble façade and travertine core—especially where had already fallen down in earthquakes—was effectively re-quarried, as it was much easier to obtain and closer to hand than anything from a quarry. Some of it was used in the construction of St. Peter’s Basilica in the 16th century. Some of it was just burned to make quicklime (useful in making steel and cement).

Contrast this to the Pantheon, Rome’s oldest standing building, which was repurposed into a church in the 600s. Or the Castel St Angelo, originally the tomb of Hadrian but rebuilt into the main fortress defending the Vatican. Neither building survives exactly as a late Imperial citizen would have seen them, but nor are they ramshackle ruins. These buildings (and there are many other examples, like the Hagia Sophia in Istanbul, or the Cathedral of Syracuse in Sicily) have managed to remain useful to their city’s inhabitants for many, many centuries, in different forms and with different uses. (You’ll note that religious buildings tend to do well with this; the Parthenon was also used as a church for many centuries.)

Buildings that aren’t adaptable—like the medieval walls of many European cities, or indeed many of the shopping malls of 1960s America—tend to just be viewed as in the way (or a source of materials for new building) and often get cleared away.