How are those nations suppost to have any quality leadership when their pre-existing infrastructures were destroyed by imperialists?
European powers maintained the local elite at their right hand and when they left, those corrupt individuals were left to rule the country. What do you expect?
Slavery pre-existed imperialism, however slavery by race was invented to justify the use of African slaves and the superiority/right to do so by European states.
States at the top cycle. 700 years ago Mali and Mansa Musa was the richest country in the world. 500 years ago the Ottoman Empire was richest, and 300 years ago China was on top. And in some time, the USA will lose it's spot as top dog.
European states recieved their initial edge in wealth during the 16th sentury when they began to plunder silver (and gold) from the New World. This drastically raised the financial situation of European countries, while it conversely deflated the wealth of the Ottoman Empire.
Europeans were able to overpower the Native Americans, in part from more advanced technology, but largely because Old World diseases (small pox) just happened to be more deadly than New World diseases (syphillis). Then the use of slave labor (at first native american, but given their poor health, turned to Africans, already immune to New World diseases) helped to bring out cash crops at low overhead costs.
And look at other places in Africa. 1830 - Algeria. France owes Algeria $$$ for grain bought on credit during all the hoopla of revolutions and wars, so instead of paying this debt, France decides to invade Algeria (originally to replace the current rulers with ones more favorable to France) but then decide to stay and colonize and make Algeria the first of the "new colonies."