Why is there an American immigrant visa consulate in Guangzhou but not in Nanjing?

Hello, embassies and consulates in China in the United States includes Beijing, Shanghai, Guangzhou, Chengdu and Shenyang. Among them, the Consulate General in Guangzhou is quite special, and it handles more things than other embassies, such as marriage visas, forwarding by China CITIC Bank, and immigration services. Not every embassy has it.