I'm really not sure about the premise of this question. When you say "impressive Iranian architecture" my first thought is the Great Wall of Gorgan. There are many many many [Çahartaq](https://en.m.wikipedia.org/wiki/Chahartaq_(architecture)). The ruins of Ctesiphon are quite impressive too.

Your example of the Hagia Sophia is a questionable comparison, because that is a building that has been maintained, restored and used until the present day.

this Encyclopaedia Iranica article may be of some help. Particularly it notes,

Sasanian architecture is characterized by the widespread use of mortar masonry and the associated vaulting techniques. Although mud brick had been developed long before, and mortar constructions were known in Parthian times, both became preeminent in the high-standard architecture of the Sasanians. Mud brick remained a most important building material (e.g. Dāmḡān, Istakhr/Eṣṭaḵr, Ḥāǰīābād, Kīš, Ctesiphon, Kuh-i Khwaja/Kūh-e Ḵᵛāǰa), and only its impermanence shifts our attention to the better preserved stone and brick ruins of Sasanian architecture.

 (a) Vaulted constructions. Sasanian vaulting techniques depend largely on the special qualities of gypsum mortar, which allows vaulting without centering because of its short setting time. Barrel vaults with “pitched courses,” the most frequent system, owe their elliptical shape and their significant step out above the impost to this technical procedure, which requires only a back wall or a narrow strip of centering for the first courses, with the following ones successively glued in front

c) Columns and other supporting constructions. With the introduction of far-spanning vaults, the use of columns as constructive elements was widely discarded. There are examples of archaizing slender columns with bases, capitals, and sometimes fluted shafts that maintain Achaemenid or Hellenistic traditions (e.g. Bīšāpūr, Nūrābād, Kīš), while those of later monuments (e.g. Bīsotūn, Ṭāq-e Bostān) reflect a fresh Western, Byzantine influence. But most often the column was transformed into a massive, round or rectangular pillar suitable for vaulted masonry constructions.

So it looks like, one, they used practical but impermanent mortar. Two, they used vaults, which are architecturally advantageous in some regards, but must be less durable than constructions like the pillars of Persepolis. If you look at the ruins of Persepolis you'll see they mostly consist of stone piled directly on stone. This is a relatively simple construction technique that is also very durable, but not necessarily practical in other ways - those columns take up a lot of space and requires a lot of massive stone hauling.

The obvious example of this taken to an extreme is the pyramids - stone piled on layers of stone. Very durable. But they leave very little usable space compared to the preposterous amounts of stone that needs to be hauled and used in the construction. I would need someone with broader knowledge of architectural history to confirm this, but it seems what you're seeing may actually reflect advances in construction techniques, allowing for more usable interior space and less labour - at the cost of being less permanent in nature. Hence why we also see many fortresses remain - there you have different priorities, of course!

I hope this was helpful! Let me know if you have followup questions.