Not sure about this particular city, but for cities in general I think it's usually because they're just absolutely destroyed. I was just listening to Dan Carlin's King of Kings podcast and in there is the poignant story of a greek general traveling through what is modern Syria I believe. While traveling through the desert the general comes across an abandoned city unlike he's ever seen, and unrivalled by any city of his time. Its 11 miles of walls were 10 meters thick and 30 meters high. There was no one around to inform him of what the city was named.

Now it is believed that the city was the capital of some (Assyrian?) kingdom, that just 200 years earlier was basically the capital of the civilised world and had stood for hundreds of years. It was destroyed by a particularly cruel horde, and everyone in it was slaughtered, the fields around it salted, the goods and artefacts taken.

It was so thoroughly destroyed that 200 years later a well read and knowledgable general couldn't even place it or find anyone to tell him about it. The only thing that remains were the basically indestructible stone walls.

I don't really have the mind for remembering history details, so I'm probably way off on some details, but I thought it was a cool story about how we've come to find these ancient ruins just all abandoned without even locals knowing what they are. Dan Carlin makes it really come alive, highly recommend listening to his stories.