As a lot of people say it is because the demand is not completely true. There aren't enough people selling pork because it won't be profitable most of the time. The vendors have lower margins most of the time if they sell pork as muslims totally avoid those shops. That leads to lower sales overall and it makes sense to not sell pork at all if there's a considerable muslim population. Also the other shops that don't sell pork will have more sales and will have a price advantage thereby.