Repeated prior conquest eroded resistance in Arab-conquered provinces but not Iran

Evidence

Yet they were absorbed, as the Persians were not. So we may have slightly modified or restated the question; we haven’t answered it. Another perhaps more plausible explanation is the political difference, the elements of power and mem­ ory. These other states conquered by the Arabs—Iraq, Syria, Palestine, Egypt and the rest—were long-subjugated provinces of empires located elsewhere. They had been conquered again and again; they had undergone military, then political, then cultural, and then religious transformations, long before the Arabs arrived there. In these places, the Arab-Islamic conquest meant yet one more change of masters, yet one more change of teachers. This was not the case in Iran. — Bernard Lewis, From Babel to Dragomans, 283