Because supposedly King Sigismund, a fervant Christian and life long enemy of the Ottoman Empire, traveled to a Muslim court and picked up a Muslim scholar as his personal physician. And that Muslim just happened to be a negro from South West Africa instead of a Berber or Arab or Turk.

Who is besieging Kuttenberg I honestly don't know. I read somewhere it's actually Henry with an army and the entire game is basically just a flashback about how Henry managed to raise and army to besiege the city. Musa, the whoremonging Muslim negro is supposedly an important ally as is his Jewish half brother from the Jewish quarter of Kuttenberg that didn't exist until the late 19th century. The entire story seems to be a big mess.

I'm not gonna play the game. I'm just gonna pretend KCD1 never got a sequel instead.