Thanks ! It is true the preservation is quite different from one thing to another. There are a bit of various reasons. Time is obviously one of them but not only. It happened sometimes that in order to build other monuments or support economy or war effort in troubled times that some monuments or places actually got destroyed to recycle the materials.

I imagine also that more modern conflicts could have damaged some places. Also, I don't remember precisely the place name, but I believe Mussolini actually ordered the destruction of old buildings to clear space and build a massive street through Rome.

If we look on the brightside however, we do have some monuments such as the Coliseum or the Pantheon which have made it quite good so far despite a few damages here and there. If we think also about the main roads paved by the romans, we are in a way still using some to this day technically, but obviously they're under concrete now..

I would also add that due to the fragmented political power and emergence of brand new regimes across Europe that the places of power changed of location too and so, I guess, the money allowed to maintain certain buildings in good shape.