Why is data so expensive in some African countries?

From buying goods to communicating with friends, mobile internet has completely changed how we live our lives. In the continents biggest cities, speedy 4G makes transactions quick and reliable. But in rural areas and poorer countries, data can be slower, temperamental and much more expensive. It means that many people are priced out of getting on line and are missing out on all the benefits that come with it. So, why is data still too expensive in some parts of the continent?