This phenomenon has a lot to do with the legacy of colonialism. With the exception of a handful of states like Nigeria, Tanzania or Cote d'Ivoire, the vast majority of African continue to have the same capitals that were established in the colonial era.

For capitals that are along the coast, this had a lot to do with access to the sea and by extension, access to trade and administrative communication with Europe. In cases like Liberia, Sierra Leone and Gabon, the colonies were set up as areas for the settlement of freed slaves from the Americas, and so cities like Monrovia, Freetown, and Libreville have their origins as freedmen's settlements at promising harbors.

For capitals that are at or very close to the border with another country, in many cases that is because the are along a major river. Kinshasa and Brazzaville are capitals that adjoin each other across the Congo river, where Henry Morton Stanley and Pierre Savorgnan de Brazza (respectively) established strategically sited trading settlements. Similarly, Bangui in the Central African Republic lies along the Ubangi river, a tributary of the Congo.

These river settlements came to be colonial centers of governance for similar reasons to those on the coast. Because they were strategically sited on rivers that were corridors for travel, they could control trade and were more accessible than upland sites that would require overland travel.

Another example would be Kharoum in Sudan, which was strategically located at the confluence of the White Nile and the Blue Nile. Though the case of Khartoum is different from other examples in that Egyptian imperialism and then later British involvement made the region into one single colony surrounding the Nile valley, rather than separate Belgian and French colonies on opposite sides of the Ubangi and Congo rivers.

There are a few exceptions to these general trends. British East Africa originally had its capital at the port of Mombasa, but in 1905 the capital was moved a railroad-junction boomtown called Nairobi, which was in the "white highlands", a rich agricultural valley that was the main area of European settlement in British East Africa. In that case, the railroads and telegraph had allowed sufficient ease of transport and administrative communication from the coast that a river site was not necessary.

Now, let me circle around to Nigeria. At independence, the capital was at Lagos on the coast. However, because of political and economic tensions between Southern Nigeria and Northern Nigeria, it was agreed to construct a planned-capital in geographic center of the country, to symbolically represent that Northern and Southern interests could meet on an even playing field.

In the case of Tanzania, the Nyerere government announced plans to move the capital from Dar Es Salaam to Dodoma in 1973. Dodoma was chosen because it is closer to the geographic center of Tanzania, and would nominally be more convenient for most Tanzanians. However, though Dodoma is the de jure capital of Tanzania, many government offices remain in Dar Es Salaam, and there is much inertia in the way of actually moving political power away from Dar Es Salaam to Dodoma.