edit the second: also Johans reason was "it's called CRUSADER KINGS" but the Muslims have crusades and kings, they just call them different, to that point the Russians as playable and so are the Germans and they have Tsars and Kaisers (though it's clear from Victoria 2 that no one at this studio knows that)
(Both Kaiser and Tsar are derived from the title " Caesar " btw)
Slightly off topic and meant as humour: you could reverse the proposition but I doubt that a game called "Jihadist Caliphs" would have sold well.