The Umayyads had a strict anti-Persian policy that was reversed by the multicultural Abbasids, who favoured Persianization. Perhaps if the Abbasids never rose, and instead Persia remained Umayyad or fell to another political dynasty the anti-Persian policies could remain, which would see the retardation of Persian language and literature vis a vis OTL.
Expanding the power and control of the Taharids might be a way to go, though of Persian stock they seemed to prefer the use of Arabic and were stridently anti-Zoroastrian. One side effect might be that it will take much longer for Persia to be fully Islamised. There would be periods of grudging acceptance, but periods of extreme anti-Persian policies. In a scenario, it is likely that a large Persian-speaking Zoroastrian underclass could develop. Perhaps in this world there is a far greater population of Parsis in India.
The religious/language difference, expounded in this world, could lead to a situation in the modern day where Persian-Zoroastrian nationalism leads to separatist warfare, or potentially a Pakistan/India-esque division of Persia into Perso-Zoroastrian and Arabic-Muslim halves.