Not a historian, but there's a few reasons why things are different today. First of all, the region was largely depopulated a few times in the last 1000 years. It first happened during the Mongol Invasion 900 years ago, in which 75-90% of the population was either killed or migrated. This conquest promptly ended the Islamic Golden Age and devastated one of the more economically prosperous and scientifically advanced regions of the world at the time (Iran, Khorasan, and Mesopotamia). Many of the large cities in Iran were entirely killed, and the agriculture of Iran was reliant on a very complicated canal irrigation system (like Rome having aqueducts) that fell into disrepair after the conquests. The next major depopulation event came with Tamerlane in the late 14th century, and he did much the same as Ghengis Khan and the region was again depopulated by 75-90%. Countries like Turkmenistan and Afghanistan were previously Iranian populations and spoke Iranian languages, but now are much more Asiatic due to depopulation events and migrations (though Afghanistan still largely speaks Dari, which is basically just another word for Persian like Farsi, Parsi or Tajiki). Iran was previously conquered by the Macedonians and the Islamic armies, but the populations and the cities were generally left alone.

The Iranian plateau, Central Asia and Mesopotamia have also gone through serious environmental changes over time, and are today significantly drier than they used to be, though is known to have been going on since the conquests of Alexander the Great.

The last major powers to come out of Iran were the Safavid Dynasty which lasted between 16th-18th century. Nader Shah overthrew this to create his own short lived dynasty and created one of the most powerful militaries in the world at its time in the 18th century. He was a big deal in the West during the 18th century, but he isn't really talked about in Western circles today.

After Nader Shah, Iran faded into obscurity as it was left behind during the Industrial Revolution. The government at the time was the Qajar Dynasty, which is overwhelmingly viewed in a very negative light by Iranians today. At the turn of the 20th century, Iran was considered one of the 5 poorest nations on Earth, and only had a population of 10 million. It was still a largely agrarian economy up until about 40-50 years ago when things slowly began to improve. The country went through huge economical reforms during the Pahlavi Dynasty and there have been pros and cons of the current Islamic Republic, but the population is at about 75 million now, dealing with rampant corruption, crippling sanctions, and very limited free enterprise.