I had the same question about concentration camps. Then I had a chance to visit Dachau during the MLK holiday as I went to Germany for a Nato conference, but due to the holiday me and my DET SGT and I had a whole day to ourselves. So we went.

Dachau, after WWII ended, was used as a refugee camp. We bombed the absolute shit out of Germany during WWII, and most of the populace was experiencing disease and famine (hence why so many camp prisoners died. The Germans literally couldn't feed their own citizens.) So after the Marshall plan rebuilt West Germany, their primary focus was on rebuilding their military. This is in the 1960s/70s, so the big fear was the Soviet Union pouring tank divisions into West Germany. So they neglected it.

Today, the camp is a mostly recreation of what it was like. The original buildings are long gone. They've built reproductions, but everyone back in 1946 had bigger concerns then preserving a camp. I get it now.

So for the answer to why did the Parthenon fall into disrepair, I bet they had bigger problems to deal with.