1. Classics has always been primarily focused on Western Civilization.
2. Leftists eliminate history because it reveals leftism to be dysfunctional and unnecessarily violent, and shows tradition and homogenous tribal culture work over the atrophy and graves of civilizations leftism always leads to with its flavor of the week mass panics in the face of reality.
3. Like with everything else, natural selection applies here. Some cultures are worthy of more study and remembrance than others.