Buddhism remains practiced in the modern world on a significant level, ancient Greek polytheism is not. In that way, Buddha statues are a symbol of something that the Taliban is actively opposed to, whereas Greek antiquities, even to the extent that they contain specific religious content, generally are not.
I suppose I don't understand where the Taliban is coming from on this (presuming it's an accurate reflection of their thought process). Both the atheism of orthodox Buddhism and the polytheism of Greek antiquities are haraam.
My guess would be simply because Buddha is a symbol of a modern religion, whereas the Greek antiquities are not.