It isn't! To get UHT milk, you have to go to a supermarket - it's an imported product. Supermarkets will also have cheese; kind and quality of the cheese vary by store.
Convenience stores only carry fresh milk. I think my favored brand is from a local dairy.
The price of milk in Shanghai, by the way, is much higher than the price anywhere in the US. This is an extreme reversal of the normal situation for food products. Imported UHT milk is quite a bit cheaper, I assume due to economies of scale, but I can't stand the taste of UHT milk.
> go to any convenience store anywhere in the developed world, and you'll find plenty of items that far less than 10% of the customer base have any interest in buying.
Mostly those items are theoretically capable of being used to full effect by 100% of the customer base. Here we have a situation where less than 10% of the customer base is even included in the potential customer base for the product. And given that milk has no place in an ordinary Chinese diet, a very large majority of those potential customers are never going to consider buying it.
I'm not sure that that's that surprising (particularly if it's UHT/shelf-stable milk); go to any convenience store anywhere in the developed world, and you'll find plenty of items that far less than 10% of the customer base have any interest in buying. As long as _enough_ people will buy it, they'll sell it; doesn't need to be the whole population or anything like it.