Hi :) Why didn’t Astapori rebuild and try to make their city more modern? They certainly had the money (or just a free work force) for it. Why did they leave Astapor to crumble in ruins ?

Underdevelopment is very common in slave societies. The logic goes like this: in the first place, slave societies by definition have most of their capital invested in slaves and the commodities that slaves produce. 

But on top of that, because capital tends to chase the highest rate of return on investment and slavery tends to be very, very profitable, you get persistent under-investment in anything not related to slavery and mono-crop plantation slavery at that. 

Hence why you have things like the Southern United States during the Civil War having only 15% of U.S manufacturing, 29% of railroad capacity, and 13% of banking, not because they were poor, but because why invest in anything but cotton and slaves Hence why you had sugar islands in the Caribbean that had to import all their food from the mainland even though the land was perfectly fertile, because no one who wanted to get rich would plant their fields with anything but sugarcane. 

So the Astapori likely let their city crumble into ruin because there’s more money to be made in investing in slaves than investing in infrastructure.