Language relatedness alone cannot explain why Arabization succeeded in Egypt but failed in Iran

Evidence

Various answers have been offered to this question. One suggestion is that the difference is language. The peoples of Iraq, Syria, Palestine, spoke various forms of Aramaic. Aramaic is a Semitic language related to Arabic, and the transition from Aramaic to Arabic was much easier than would have been the transition from Persian, an Indo-European language, to Arabic. There is some force in that argument. But then Coptic, the language of Egypt, was not a Semitic language either, yet this did not impede the Arabization of Egypt. Coptic survived for a while among the Christians, but eventually died even among them, except as a liturgical language used in the rituals of the Coptic Church. — Bernard Lewis, From Babel to Dragomans, 280

Categorization

• QUESTION: The author rejects the language-family explanation but hasn’t yet offered his own mechanism — worth following where the argument lands next.