Slave Trade: Why Africa?

A question from a new published paper by Angeles is: Why only Africa?  

Figure 1. African slaves embarked to the Americas

One aspect that set Africa aside from Eurasia during the early modern period was a higher degree of cultural fragmentation. By this we mean that cultural areas, regions within which people would share some essential cultural elements and recognize each other as similar, were smaller in Africa as compared to Eurasia. How this would translate into lower costs for obtaining slaves is not very difficult to see. If we assume, in accordance with Finley, that the enslaving of one's own people was pretty much forbidden in all societies then would-be slave traders need to run their operations against societies other than their own. Eurasia's large cultural areas meant that this required long-distance operations and large-scale military actions, rendering the capture of slaves very costly. Africa's cultural fragmentation, on the other hand, implied that raids of even a few dozen men attacking villages from a nearby region would be a cheap and acceptable way to obtain slaves.

That sugests that Africa was much more culturally fragmented than India, for example. 
The abstract:

This paper offers an integrated analysis of the forces shaping the emergence of the African slave trade over the early modern period. We focus our attention on two questions. First, why most of the increase in the demand for slaves during this period came exclusively from western Europeans. Second, and of most relevance for present-day development outcomes, why was the overwhelming majority of slaves of African origin. Technological differences in manufacturing technology, the specificities of sugar (and other crops') production, and the cultural fragmentation of the African continent all play a role in the analysis. Supporting evidence for each of our claims is provided from a broad corpus of relevant literature.