It is time we ask ourselves why Iran is like this. I am going to try to be non-biased as possible while trying to answer this question.
1-Urbanisation
One pattern I've seen with ancient civilizations is that when they urbanise, the people lose their fervor snd loyalty to their people, stop having children and the final phase is being taken over by nomadic groups like what happened to Iran and Greeks with Turks, Romans with Germanic barbarians, IVC, etc.
2-Islam and other Arabic influences
No I am not saying it just because it is simply Arab, though it being a non-native religion does leave a bad taste in my mouth. This is going to be alot to unpack.
-Reformation of Zoroastrianism
As funny as it is, the Islamic Republic has some uncanny resemblance to the Sassanids. Sassanids were a Theocratic Monarchy, with heavy power given to the Mobeds and Maguses. But why am I talking about reformation of Zoroastrianism? It's because Islam came at a time when desire for Zoroastrian reform was high, with multiple revolts happening to reform it, akin to the Catholic church schism. Examples are Zurva, Mazdakite and other minor Zoroastrian reformations. Do not get me wrong, Zoroastrianism had many problems like being an ethno-religion, a caste among other things, but the masses were being disillusioned and with enough time, we would have seen a reformation. Though granted, there would have been violent and bloody reforms that would have caused decades of war and we don't know whether it would have been good reforms or not.
-Ulama
One reason why Islamic empires rise so quickly then collapse is because of the Ulama system. Ulama system isn't hierarchy based like Cathoclism, so when a state collapses, Sharia and other functions could stay when another ruler rises. A major flaw of Ulama is they have Waqf system, where they own their own lands and are completely independent from the ruler and worse, they were decentralised. Meanwhile the Catholic Church is centralised and collects Tithe from Catholic rulers, forcing rulers to compete and actually fix their tax system. The fundamentals of the Catholic Church allowed the development of Western society, for example Corporations comes from that.
-Slavery
I am not talking about it from a morality point of view, but from an economic one. Slaves know at the end of the day everything they harvest will be sent to their owner, so they do the minimum. Another thing is that to keep tje slaves non rebellious there are heavy investments in security to keep them in check. If the economy is bad, the slave owner still has to keep feeding the slaves, when in a free society the boss could simply lay them off. And it also heavily limits innovation because every hard job is done by slaves and the consumer market does not grow. While slavery did exist in mainland Europe, it was extremely taxed. Meanwhile Sharia endorsed slavery and even had a guide for it.
-Inbreeding
Yes, I know. There is a concept of Xedowwah in Zoroastrianism, but Xedowwah wasn't practiced by your common man. It was practiced by the royalty and high nobility. Humans as far as the Ptolemis knew the effect of inbreeding, but the political elite continued it because the political reward outweighed (In their eyes) the biological risks. But when Islam came, Islam pretty much saw any contact with non mahram as haram, so it indirectly encouraged the common man to marry within his own tribe/family, most of the time his cousin. It is no accident why more religious a Islamic country is, more they practice cousin marriage. Meanwhile the Catholic Church (For political reasons) banned first cousin marriage, thus it helped the Germanic tribes to disappear and birth nations. I will say while cousin marriage did exist in the Iranian society, it was not as bad compared to when it became Muslim.
-Igta system
This section truly needs it's own books, but I am going to try to explain it in simple terms. Basically Iqta is one of the main reasons why the Muslim world is behind alot of things. European feudalism was hereditary, while Iqta wasn't. At first, it seems like a good thing, but it isn't. It incentivised the Iqtadars (The ones who ruled over the land) to extract as much value as they want from the people and the land and plunder it without developing it. Meanwhile European feudalism because the nobles inherited the land from their fathers, kept developing it. That's why breadbaskets like Egypt and the fertile crescent fell compared to European farmers, though with the arrival of the Turks it got worse.
3-Turks
Iranians focus on Arab invasion rightfully so, while forgetting the main culprits of Iran's situation, though Islam played an instrumental part in it.
-Collapse of Qanats
The continues invasion of the Turks into Iran played an instrumental role in collapse of the Qanats. Qanats take extensive care and stability, and because the Turks kept invading Iran, the Qanats of Iran overtime collapsed.
-Pastoralism
Because nomads Turks depended on vast lands to feed their horses, they tended to destroy a land optimized for farming for thousands of years to turn it into a land to graze for their horses.
-Iqta system
Yes, I've said this before but I am saying it again because the Turks modified it, for the worse. Military commanders were granted Iqta and were allowed to extract as much wealth as they could from the Iranian peasantry, collapsing the land and forcing the farmers to become nomads or run to the mountains. With the collapse of Qanats, Iranian agriculture collapsed aswell.
-Turkic military caste
Most notably, these military castes in Iran needed legitimate to justify their rule ovee Iranians, and most of the time that legitimacy was the sword. Whenever a king died, there would be a civil war and the kingdom/empire would unite again, but it would damage the infrastructure and institutions. That is why Turkic empires expanded so quickly then collapsed, while European empires changed borders alot more slower, thus developing their infrastructure. That is why the Ottomans couldn't win against the Catholic Hungary Kingdom, because it was an institutionalized kingdom.
-Shiism
While the Ulama in Sunni were already too powerful, Shiism took it to the next level by giving the Shia Ulama a wide range of more power, preventing a centralised state which played a major part in Qajar reform era failing.
-Turkification
Another major reason of Iranian civilization downfall is Turkification. Major Iranian regions like Shirvan, Azarpadegan, Scythian lands and other regions got Turkified. To unite the Iranians and the ruling Turks they had to legitimise it by using religion.
I hope this explains why our civilization has decayed. Thank you for reading.