Well, Arabism succeeded where Arabic did, as someone pointed out. And Arabic succeeded where Arab tribes settled. Mountains were not good place for Arabs and usually were more independant of the lowlands, hence the part of the Arab conquests that weren't arabised is what's in Zagros mountains and beyond (although there are some Arabs in inland Iran and even in Afghanistan!). Also, it was quite easy for Iraq, Syria or Egypt to get arabised, because Aramaic, and to lesser extent Coptic, were quite similar to Arabic. When it comes to Maghrib, it got arabised quite late, only after the invasion of Banu Hilal Arabic tribe in XII century I believe, mostly, and even today the process is not complete. Take into account also that pro-Abbasid revolt in Khurasan was an indigenous movement and brought Iranians back to rule to some extent in the eastern regions, and not so long afterward different dynasties of iranian origin spread there, so actually the Arab rule was quite short there. The same goes for Maghrib, as Arabs were mostly just leading Berbers there after they converted, and the states of Rustamids and Idrisids - even if Rustamids were of Persian, and Idrisids of Arabic origin, and Arabic was the language of culture, were actually to much extent Berber-speaking. Which explains why these lands were not easily arabised. Also, they are simply more far away.
Also, take into account that Arabs started settling in Syria and Mesopotamia long before islam, so these lands were already full of them when they were conquered, even if they were Christian Arabs.