>>17919889 (OP)
Because Egyptians, Levantines, Berbers... already spoke Afro-Asiatic languages so the masses were easily able to learn Arabic as a 2nd language, and eventually adopt it as their first.
Non-Afro-Asiatic Muslims (Turks, Persians) spoke completely different languages so Arabic never spread beyond educated elites.
Also, there was no "erasure". Arabs never forced non-Arabs to speak Arabic. They didn't even force Christians/Jews to convert to Islam.
Arabic simply spread by virtue of being Islam's official language. Every Muslim tried and still tries to learn Arabic to be a better believer. To be able to read hadiths, to understand islamic theology, etc. Afro-Asiatics just had an easier time learning Arabic since they already spoke similar languages.