Because selling media costs money, and some countries can't really offer much reward. To sell music you have to deal with labels and, sometimes, local authorites, while risking small profits. Here in Poland we have some archaic organizations like ZAiKS, and most people view paying for music as insanity. That means that we have no major mp3 store in Poland at all (afaik 7digital and Beatport sell to Poland, and we have some niche stores, but that's all unless you want to import scrap cards)