Why Sassanids Failed To Win Against Arab Invaders?

This has always been a question asking, since the fall of Sassanids. Why did Iranian get defeated despite of having more troops against fewer Arabs fighting with their amateur weapons, that the Iranian troops mocked them because of their elementary weapons(like sewing machine).

Like many other facts it just didn't happen for one reason. I think there were a series of incidents that led to this.

First, it was the severe theocracy that the Magians were almost ruling out the country. One main difference between Achaemenids and Sassanid is that Cyrus, Darius, Xerxes and Achamenid kings were secular and easy going with people's religion. They conquered countries but never tried to turn them into Zoroastrians. They even respected their beliefs, as we might see in Darius's entering the Egyptian's temple, or Cyrus's conduct to Jews, or even Xerxes, while he praised the Greek temple. That was one reason to Achaemenids historical eternity.

One other thing was the fights against Romans and Christians that time, that weakened the country and provided a condition that Arabs almost didn't confront any resistance from one of the greatest army of those times. Iranians were ready to confront any danger from Romans but not from the powerless Arabs.

People were almost forgotten by their rulers. Especially, after the one last greatest king, Khosrau II, Khosrow Parviz, the dynasty seems to go down, creepingly.

People welcomed Islam as a cosmopolitan, idealistic religion. They were so tired and desperate of Zoroastrianism. Some say if East Romans were more ambitious and invaded Iran earlier than Arabs.

Then, it wasn't a classical war. Like we see these days. The Muslim invaders came to kill or get killed. In both cases they were said that they'll go straight to heaven.

We can also consider some bad weather too. In Qadisiyya battle ground the wind was against Iranians. Although there was one man fighting and encouraging the soldiers to hold on.

Rostam Farrokhzad, a very brave heart, nationalist general, stood fearlessly against the Arabs. After his defeat, in Qadisiyya, everyone was expecting Iranians defeat.

Since then there is a never ending hatred and enmity between Iranians and Arabs. Iranians think of that as a loss that shouldn't take place. And Arabs are proud of that. Although they pretend to be like one Muslim nation.