Convention.

It's not uncommon for academic history books to have a disclaimer in the front about how the author elected to use the Anglicized (or Latinized) names of important figures where one exists, but then tried to stick with transliterations of the original in other cases. The use of the Anglicized/traditional name is to aid the reader in easier recognition.

Thus we also read and talk about Frederick Barbarossa, Augustine, Jesus, Copernicus, Averroes, etc. The same standard is often applied to place names as well. English-language histories of Germany will talk about Munich, not München.