Absence of Roman rule made Hellenistic influence shallow and transient in Iran

Evidence

Iran was never conquered by Rome, and therefore the cultural impact of Hellenistic civilization in Iran was much less than in the countries of the Levant, Egypt and North Africa, where it was buttressed, sustained and in a sense imposed through the agency of Roman imperial power. The Hellenistic impact on Iran in the time of Alexander and his immediate successors was no doubt considerable, but it was less deep and less enduring than in the Mediterranean lands, and it was ended by a resurgence, at once national, political and religious, and the rebirth of an Iranian polity under the Parthians and then the Sasanids. A new empire arose in Iran which was the peer and the rival of the empires of Rome and later of Byzantium. — Bernard Lewis, From Babel to Dragomans, 284