This is a sequel to my post Why did the Protestant Reformation happen where it did?.
I play Crusader Kings II. At many of the start dates, Egypt and Arabia (a Sunni Area) are controlled by the Shi'ite Fatimid Caliphate. Meanwhile, Iran (a Shi'ite area) is controlled by the Sunni Seljuk Sultanate (or at later start dates, by the Sunni Timurid Empire).
In that game, the AI often converts the populace as quickly as it can. But in real life, the Fatimids, Seljuks and Timurids failed to get the majority of their subjects following their branch of Islam. Is there a specific reason why Sunni Islam took root and held its grip so strongly in Egypt and Arabia, and why Shi'ite Islam took root and held its grip so strongly in Iran?
On a different note, Turkey and its predecessor, the Ottoman Empire, were majority Sunni nations, with a significant Shi'ite minority. But why both forms of Islam take root there, considering that 1000 years ago, most of Turkey was a Greek Orthodox Christian stronghold with negligible Islamic populations?
Source: reddit post