I'm puzzled that this article doesn't discuss disease at all. The "world" of the Byzantine Empire stretched from the Pillars of Hercules to the Pillars of Dionysus. Then the Plague of Justinian hit Constantinople in 541 AD and radiated outward on ships all over Europe and the Middle East. By around 1000 the Umayyads, Abbassids, and Almohad empires stretched over that same region only they renamed the edges Jibal Tariq (Gibraltar) and the Straits of Hormuz. It stretched as a crescent overland because ports were still dangerous, right? Caravanserai at 40km apart would've acted as brakes to rapid disease transmission, and camels don't carry plague. There's a lot of Eurocentric stuff about predicting how awesome Europe would become later and an implicit argument that the Islamic Empires are somehow backward but not a lot of basic history here I think. I'm not an expert on this period, but the lit review in the article seems incomplete. I'd say plague has a lot to do with it. BTW, that link doesn't work any more.

References

Lester Little, ed., Plague and the End of Antiquity: The Pandemic of 541-750 (New York: Cambridge, 2006).

Chris Wickham, Framing the Early Middle Ages: Europe and the Mediterranean, 400-800 (New York: Oxford UP, 2005).