[jkuria]

I believe Jared Diamond answers this question in Guns, Germs and Steel. Africans didn't have iron tools (or acquired them relatively late) and so were not as effective hunters. The animals therefore had a much longer time to adapt against their predators.

[blackbagboys]

gwern rejects Diamond's hypothesis later down the thread, so I'm interested in hearing about any alternate hypothesizes he may have.