I wonder whether there would be also other options, but one very important aspect is the smog: the city center is where many industry-relevant things meet: the factories are all around it, transport, cars... and since it is surrounded by bad air, the air takes longer to dissipate. This is especially visible if the city is in a valley, such as Mexico city foe example.
I believe that, He divided the empire into two sections.
The media overdoes everything. The media made people terrified which caused harm to Vietnamese culture.
Spain, Hope this helps :)
