>>17966571
Environmental determinism. Trade flows easier from and x axis than a y axis. The crops you can grow in Spain you can also grow in Turkey, but will have trouble to grow them in England even tough they are the same distance. The Sahara and the forests of Africa also sucked to travel through. Europeans just didn't bother going there and spent most of the Middle ages waring and conquering the arabs and pagans. This further allowed a transmission of ideas and trade.