Why does Africa have so many languages?
csmonitor.com
csmonitor.com
Why is that an insightful question? I wouldn't have expected a strong correlation, and from the article, there doesn't appear to be one.
At the risk of being too dismissive, I thought the reasons for linguistic diversity were already well known (that is, physical and political separation and a lack of trade/communication).
If you combine just Mayan, Nahuatl, Mixtec, and Zapotec you're already in the millions.
I also think that the question suffers by failing to compare the range of languages found in other regions. The geographic distribution of the Indo-European language facility is comparable to Africa and contains around 500 known languages, though it is likely that at least as many were extinguished through various means (peaceful and not) over the centuries. Linguistic diversity is the norm, not the exception, though large-scale political consolidation tends to mask this.
There was however a few languages that everyone spoke. Tok Pisin was a pidgin language spoke by almost everyone, and English (due to it being the language used in schools) ran a close second.
Despite the relatively few languages spoken in Europe, we don't have a universal dialect that everyone understands. The vast majority of us are monolingual.
Isn't that the role filled by English?
Just from personal experience, people in the nordic countries generally have a near-native command of english. In French and Germany it's a bit hit and miss, but younger people generally speaks english well enough to be able to communicate. In Italy it's pretty bad though, you can tell that just by all the signs with engrish.
High levels of synchronic linguistic diversity should be considered the pre-modern norm, even aside from the usual troubles of distinguishing languages from dialects. And nation-states are very common these days.
The answer is probably mostly language standardization, promotion of elite lects, network effects, and official promotion of the standard language, often alongside the abuse of minority language communities.