Demand. Chinatown can do it because it's ethnic residents don't just want a great selection of produce, they demand it. They want it bad enough to fuel an entire alternative supply chain. If the people buying Chinatown's produce were replaced by less-discerning customers, the produce markets would disappear overnight and the farmers would have to unload their current crops at cut-rate prices.
Normal customers don't want it that badly. So supermarkets can't rely on being able to continually turn a profit on such a risky endeavor.
A service doesn't just have to be ten times better than the alternatives to drive mainstream adoption. It also has to be something people want bad enough to pay to create an entire industry around delivering it.
This still doesn't explain why it is cheap -- it's cheap likely because there are not too many steps between the consumer and the producer? And not too much money spent on infrastructure...
I bet the produce wasn't so cheap at first. Economies of scale had to get wrung out over decades. This is where the "China" in Chinatown becomes important, the Chinese really value the sort of long-term thinking that makes this kind of endeavor profitable.
Normal customers don't want it that badly. So supermarkets can't rely on being able to continually turn a profit on such a risky endeavor.
A service doesn't just have to be ten times better than the alternatives to drive mainstream adoption. It also has to be something people want bad enough to pay to create an entire industry around delivering it.