Looking at a map of where Arabic is the native language, I notice that Afro-Asiatic languages were already spoken in most locations (sometimes alongside Latin or Greek) before the Islamic conquests. Conversely, Arabic today is not the native language in places which had unrelated languages beforehand: the Caucasus, Iran, Iberia, and part of south-central Asia.
On the other hand, I notice that most Arabic-speaking regions are arid or semi-arid.
I would like to know if either of these are significant, or if they just coincidentally overlap what was easiest for the caliphates to take over and govern.
Wondering about Arabic in particular may miss the point, with a better question perhaps being why Afro-Asiatic languages are mostly spoken (natively) in arid and semi-arid areas.
I have a related question: how mutually intelligible are the regional dialects of Arabic? I've heard that it can be extremely difficult for a Moroccan to understand a Yemeni, for example, and I'm wondering how the history of the spread of Arabic (by colonization vs conversion) is reflected in today's dialects.