Lara Novakov (ლარა ნოვაკოვი) via Quora answers the question why Iraq Arab but Iran Persian.
According to the analyst;
”I hope that this is not a confusion between Iraq and Iran because the two are different countries and different peoples.
Second, as someone already mentioned here, there were already Arabs present in southern parts of Iraq – the Lakhmids who were present there from 300–602 AD and Ghassanids who were the “neighbors” of the Lakhmids. Ghassanids were Arabs who migrated to the Levant region initially and then mixed with Greek Christians and most of them adopted Christianity in the process – so the majority ended up being Christian Arabs.
”Kingdom of Araba was a 2nd century kingdom which was located between the Parthian Empire and the Roman Empire – Arabic was spoken in the kingdom and it was ruled by an Arabian princess. Many say that it is the first Arab state to be established outside of Arabia and in parts of modern-day Iraq.
Iraqis were in a weird way always related to the Arabs – because they were Mesopotamians and Semitic speaking peoples and after the conquest of Iraq they were Arabized (as in adopted the Arabic language as their main language, as well as customs and traditions) which changed as time passed and they eventually adopted an Arab identity as a whole.
”My personal opinion – Iranians always had a stronger sense of identity, from the very first Persian Empire (Achaemenid under Cyrus the Great) to the last before the conquest (Sassanian). They always maintainted and focused on maintaining the Iranian culture – Persians were one of the many ethnic groups living in the Persian Empires along with many others.
How did Iranians manage to resist Arabization? First, the strong identity they developed played a huge role in it – but it was not the only factor. Some others include the conversion to Shia Islam from Sunni Islam during the Safavid dynasty (and Ismail I specifically) was the process of conversion and making Shia Islam the official religion of the Empire, which is also regarded as one of the turning points in the history of Islam. Then you have Ferdowsi who wrote the Shahnameh which is the longest epic poem written by a single poet. He is often regarded as a savior and one of the greatest contributors to the Persian language.
Multiple factors played a role as to how Iranians kept their identity after the conquest – if you ask me, the fact that they had a strong identity much before the conquest itself is one of the biggest factors.”
In addition to that, another user, Hatim Ghadhban Abood said;
”In addition to the Lakhmids, there were many Arab tribes (like Rabiaa) in the west and the far north (Mosul & Jazira) of the current Iraqi map, but they were travelling every winter to the warm fields south. Then, in spring and summer they had been travelling again north close to the nowadays Turkey boarders. Later many of these Arab tribes involved in Christianity and became a part of the Assyrian population Northern Iraq.
Those Arab tribes considered to be the early “unofficial” Arab population of Iraq (in contrary to the Lakhmids kingdom).”