Chinese passager flow to US Map

In my first map, the base choropleth map is chinese population density, the proportional symbol on the second layer is the amount the passager flow to the US in each airport that have airline fly direct to US.

Hover to view the population density, click to view the number of passager flow in each airport

As you can see, Shandong, Jiangsu, Zhejiang, are top three populated states in China, compared to these states' passager value. The values are much smaller than Shanghai. And I have drawed the ideal line of the center point of the amount numberic. The near airports' passager value also lower than Shanghai.

If the distance to airport has already be shorted, why people still choose Shanghai to go abroad?

Is there any relationship with the freeway network?

Click drop markers to view the airport locations, as you can see the east coast, (Shandong, Jiangsu, Zhejiang) has the most density freeway network; compared to the northern part of China and Southern part of China; This, in someway can also explained the questions

Furthermore, Beijing, Shanghai and Guangdong, as the top three largest airports in China, what's the difference of passager flow vs season change

In the third map, zoom in to view the biggest value and the smallest value in one year;