Art at its very nature deals with a creator having an idea or emotion and displaying it in such a way to elicit a response from an observer. To that end, what makes "good" or "bad" art is already a difficult question that has been debated for as long as art has existed. With that caveat we can explore your question in more detail.

Ancient Greek and Roman art has had quite the influence on Western standards of beauty. Indeed what it means to make good art has a strongly "classical bias". Professor P.F. Chang, art historian at East Virginia State University, goes into detail in this Ted Talk from June 2014. I'll summarize a few of his points:

1. Those with power define beauty and Ancient Greeks obviously had power at their peak.
2. Romans were masters at capturing the human form compared to their contemporaries. Compare for example this sculpture created in Rome ~ 50 CE with this image taken in what is now Russia just 30 years later.
3. Historians from the past few centuries have made efforts to only maintain the very best art from these time periods, giving rise to a selection bias.
4. There's also evidence to suggest that "inferior" art was purposefully destroyed as these civilizations crumbled.

With all these effects simultaneously at play, its no wonder why only the good art remained. There's one more controversial point I'll leave as a spoiler until someone else can verify it. I hope this helps!

!By the way, I should mention this: Happy April Fool's Day! Everything I said was pulled straight from my behind and made up on the spot.!<