Why was the cultrue of the Hellenistic period so different from that of classical Greece?

The Macedonians introduced an overlay of Greek culture on the peoples of disparate areas - Libya, Egypt, Palestine, Syria, Persia and, initially at least, as far as Central Asia and the Indus. Under this overlay continued the previous cultures, languages etc, and there was a progressive reversion to them, with only the aristocracies maintaining the Greek culture. Then came the Roman takeover, and it became even more muddled.