One reason is because Yunnan is the only place in China where coffee is grown, so the vendors have bulk supplies and can roast and grind to order. For some reason Chinese importers don't import the pre-ground vacuum packs that you can get at any grocery store in the west, they only import bags of whole beans.
The other reason is that Yunnan is nestled up next to Vietnam, which means couriers do not need to route around Hubei to get their wares to Guangdong.
In case the news hasn't made it clear: Wuhan is essentially the hub of China. It lies directly between Beijing and Guangzhou on the north/south axis and Shanghai and Chongqing on the east/west axis. It's the worst possible place for a virus to break out, logistics-wise.
We'll see if this plan works out.
If not, there will be a lot of Nescafé and misery in my future.