Why doesn't every adaptation keep characters the same race as they were in the source material?
People like what's familiar to them, and contrary to what some may think, it's far from limited to white-washing. India remakes Hollywood style films all the time to appeal to the Bollywood crowd, and naturally casts Indian actors in all roles. Japan routinely makes live action versions of anime featuring Europeans, but using exclusively Japanese actors. Some nation will even edit a film to change the tones of existing actors within the film, or cut characters completely based on biases.