Some time ago, I wrote about why Egypt speaks Arabic and not Egyptian. In it, I touched on why Arabic replaced other Semitic languages in West Asia and North America.
Why Arabic replaced other languages
The reason is that Syriac, Amazigh, Coptic and other Semitic languages were similar enough to Arabic to assimilate or be supplanted by it.
This assertion is based on something I read in Israel Wolfenson's book on the history of the Semitic languages. Wolfenson was a professor at Cairo University in the first half of the 20th century, and even adopted a kunya of Abu Tho'ayb أبو ذؤيب which means "Father of a little wolf" derived from his last name.
Kazem Barqanisi affirms this theory
Now, another linguist confirms this finding. He is Kazem Barqanisi, as interviewed by Al Jazeera (in Arabic). He makes a distinction between Farsi and Turkish on one hand, and the Semitic languages on the other, and explains why Farsi and Turkish persisted, while the others got supplanted by Arabic.
Farsi most influenced language
Farsi, he says, is the most language influenced by Arabic by borrowing terms from it. Yet, an Arab can read a Farsi book that has 90% of its content borrowed from Arabic and not understand much, because of the different grammar of the two languages.
Apart from being the language of government, literature and learning, Arabic was also the language of the Quran, and hence religious life and law.
Why Arabic died in Iran
He also mentions that despite Arabic being a second language for the Persian populace for centuries, it has not "stuck" with the population. Not because of nationalistic tendencies, but because of Farsi being an Indo-European language, while Arabic is Semitic.
Purification of Farsi
The nationalistic tendencies in Iran do exist though, ever since the 1930s, with some calling for purification of Farsio from Arabic influence, and replacing terms that are of Arabic origin with new invented terms. Barqanisi says these attempts are doomed to failure.