Historiography tends to focus on the precedents of events that are considered as particularly important, to work backwards in other words. To take an example from a widely different field, many people have studied under countless angles the history and the archaeology of the Old Latium in the 6th - 4th century BC. In itself, it is not particularly interesting; no more, at least, than other areas of Italy in the same period (and in fact certainly less than others in terms of urban development; Magna Græcia was probably much more impressive). However, this is the area on which the imperial power of Rome would grow, and it naturally attracted the attention of historians. Similarly, there is in fact no objective reason to think that the development of the Mali Empire is less important than, say, the construction of the Grand Duchy of Moscow, but the successor states of the latter would become imperial powers, while the areas of the former would be deeply reshaped by the influence of colonial empires (I personally think that using history to expain modern events is a misguided view and that history, not unlike ethnology, is essentially a formation in relativism). On the basis of this main idea, I would therefore tend to think that two main factors should be singled out:
Contemporary African events can largely be explained in terms of post-colonial and colonial history (even if many patterns of tribal history or tensions between neighbouring regions with different environments do find interesting parallels in pre-European history).
Africa itself is widely considered as peripheral, a fact that naturally leads to a disinterest of the public for its pre-colonial history.
Another factor, to be fair, is the difference in the nature of sources — African history in the pre-colonial period relies on external sources (often from the Islamic world), oral traditions and a burgeoning archeology, all of whom are not as easy to use as a good old narrative histories from the European tradition.