There's the scale of economics. If only few people buy healthy food, supermarkets have no incentive to offer it at all. Even if they offer healthy food, the price is higher because they can't buy in bulk or else they waste much of it.
Then how come most grocery stores have healthy food?
If it truly was a minority group that wanted to eat healthy food, what right do they have to force the majority to buy what they want to make it more economical? Although the premise is flawed, because the real reason 'healthy food' is expensive is because the foods considered healthy inherently cannot use many methods that increase yield like pesticides. And then some go so far as to claim GMO's are unhealthy/cannot be organic, further driving up the prices of foods that get to wear the healthy food labels. Increasing demand for healthy food would make it more expensive, certainly in the short term.
All over America there are many areas known as "food deserts" because stores do not stock fresh foods and produce. In those areas the only food readily available is shelf-stable, highly processed, very high caloric density junk food.
Now think about being on a Pacific island where leaving your food desert is not just a matter of a long drive or bus ride down the road to a different community.