Because lack of food.
This is a list of countries and dependencies ranked by human population density, and measured by the number of human inhabitants per square kilometre or square mile. 1 Antarctica is a continent of 14,400,000 km 2 in area with territorial claims from multiple countries that are not included elsewhere in the above table. 2  With a population of roughly 1000, this results in a population density of ~0.00007 people per km 2 (much below Greenland's density of 0.026). The most densely populated country of Africa is ----- Rwanda. Nigeria, has the highest population (146.2 million) in Africa, but is not necessarily the one with the highest population density (per area). The most densely populated city in Africa is Cairo in Egypt with an approximate 14,000,000 people. Rwanda has to be the most densely populated country in Africa. It is only 26,338 square kilometres and it's population is over 11 million. Edit: Comparative statements are prone to bias, especially when borne out of assumption. In terms of numbers, the population density of Kigali is about 1000 per sq. km Population density (in agriculture: standing stock and standing crop) is a measurement of population per unit area or unit volume; it is a quantity of type number density. It is frequently applied to living organisms, and particularly to humans. Main article: List of sovereign states and dependent territories by population density. For humans, population density is the number of people per unit of area, usually quoted per square kilometer or square mile (which may include or exclude, for example, areas of water or glaciers). Commonly this may be calculated for a county, city, country, another territory, or the entire world Map of African countries by population density. This is a list of African countries and dependencies by population density in inhabitants/km². Saint Helena, being closest to Africa, has been included Nigeria is by far the most populous nation in Africa and the Middle East and its population is nearly double that of Egypt which comes a distant second in terms of population size in Africa. Posted on April 10, 2010 by beegeagle. The most outstanding characteristic of Nigeria is arguably its very large population of over 150 million people spread out across a vast and geographically very diverse area of land nestled between the Atlantic Ocean to the south and the Sahara desert to the North. 1 The most densely populated sovereign nation is Monaco, with a population density of 16,754 people/km 2. 2  Antarctica is a continent of 14,400,000 km 2 in area with territorial claims from multiple countries that are not included elsewhere in the above table. 1 Antarctica is a continent of 14,400,000 km 2 in area with territorial claims from multiple countries that are not included elsewhere in the above table. 2  With a population of roughly 1000, this results in a population density of ~0.00007 people per km 2 (much below Greenland's density of 0.026). Among the African Countries, Namibia is least densely populated. Below is the map of Africa showing the top ten countries having lowest population density along with the table stating their capital city and density in square kilometers and square miles. mid africa is very sparsely populated .. it includes Congo, tanzania, kenya, angola. These areas are under-developed, lack of food.. more-over these areas have a good wild-life and hence leading into a sparce africa. 
https://simple.wikipedia.org/wiki/List_of_countries_by_population_density http://www.answers.com/Q/The_most_densely_populated_country_in_Africa https://en.wikipedia.org/wiki/Densely_populated https://en.wikipedia.org/wiki/List_of_African_countries_by_population_density https://beegeagle.wordpress.com/2010/04/10/why-is-nigeria-so-densely-populated/ https://simple.wikipedia.org/wiki/List_of_countries_by_population_density http://www.mapsofworld.com/africa/thematic/countries-with-lowest-population-density.html https://nz.answers.yahoo.com/question/index?qid=20080407053917AAnbRhW
why is africa densely populated