Jim Robinson and Ragnar Torvik make an amazing observation in a new NBER paper:

at the time of independence, parliamentary constitutions outnumbered presidential constitutions 4 to 1 in Africa. Yet in country after country there was a switch towards presidentialism. …only three of the 21 countries which started with parliamentary institutions have not changed them, and two of these – Botswana and Mauritius – the only two countries which have been economically successful in Sub-Saharan Africa since independence.

Their explanation:

Our analysis is predicated on two ideas: first, that minorities are relatively powerful in a parliamentary system compared to a presidential system, and second, that presidents have more power with respect to their own coalition than prime ministers do.

These assumptions imply that while presidentialism has separation of powers, it does not necessarily have more checks and balances than parliamentarism. We show that presidentialism implies greater rent extraction and lower provision of public goods than parliamentarism.

Moreover, political leaders prefer presidentialism and they may be supported by their own coalition if they fear losing agenda setting power to another group.

What makes Africa special?

As compared to countries in Western Europe or islands in the Caribbean, which have sustained parliamentary constitutions, the preferences of different political salient groups in Africa, for instance, are much more polarized. Political parties are often highly regional…

It is this which raises the stakes from agenda setting and makes the majority prefer to have a president to make sure that they cannot lose agenda setting power to the minority.