Throughout a lot of Roman and early Medieval history Antioch seems like it was a very important city. What happened to it that made it not recognized in modern times?
In 1254 the duchy of Antioch effectively became a vassal of the Armenian empire. The Armenians had drawn up an alliance of sorts with the Mongolians. The treaty was that if the Armenians helped with the war against the Mamluks (the muslim sultanate that ruled over egypt) that Jerusalem would be ruled by Christians.
The war against the Mamluks went well at the start, with the Mongols making it all the way to Damascus. but, at the battle of Ain Jalut, the Mongolian and Armenian forces were defeated. this allowed the Mamluks to retake their land in the [Levant] (https://upload.wikimedia.org/wikipedia/commons/2/28/Levant.gif) and threaten Antioch.
The Armenians asked the Mongols to help relieve pressure from Antioch but all that the Mongolians did was attack frontier fort of Al-Bira, which is approximatly 15 km north of jerusalem. When the Mamluk forces took Antioch, they either killed or enslaved every Christian in the city. This act of religious vengeance reduced the population of the city.
Another reason why the city of Antioch had declined was due to the disintegration of the Silk Road. The Silk Road had been a major trade route between Europe and Asia for a long time, and had seen a rejuvenation during the time of the Mongols, as the low rate of crime in the Mongol empire encouraged trade. As the Mongol empire Fell, the security of the silk road could no longer be counted on, reducing the trade along the Silk Road.
With less trade going through the city, Antioch could not support such a large population both in raw material and economically, as the economy of Antioch would have been mostly a service economy to the traders moving through.(brothels, food warehouses, intermediary traders, ect.) Antioch was also in a seismicly active area, which made living in the area more difficult. By the early 15th century the city of Antioch was effectively uninhabited.
TL;DR: Due to a combination of Pillaging, slavery, reduced trade, and other factors the city of Antioch was left abandoned and forgotten.
Just so no one get confused: Antioch is still around. It's not a lost city, just a shadow of its former self.
Nowadays it's known as Antakya...which is Turkish/Arabic for "Antioch".
Follow up question: how much of the city still remains? If I happened to be a tourist heading to the region, is there enough left to rent a car and drive out to see?
