This was partly because the Islamic Empire relied more and more on soldiers from the frontier, in this case the Turks, and also slaves pressed into military service in order to be the backbone of their army, a strategy that has been tried over and over again and has worked exactly zero times, which you should remember if you ever become an emperor. Actually, our resident historian points out that that strategy has worked, if you are the Mongols. More important than the Persian-style monarchy that the Abbasids set up was their openness to foreigners and their ideas. That tolerance and curiosity ushered in a golden age of Islamic learning centered in Baghdad. The Abbasids oversaw an effervescence of culture unlike anything that had been seen since Hellenistic times. Arabic replaced Greek not only as the language of commerce and religion, but also of culture. Philosophy, medicine, and poetry were all written in Arabic, although Persian remained an important literary language.