for most anime and manga, it is because a majority of japanise and those of asian decent are of a more 'white' colour than anything else and a majority of the lead charicters of japanise animes tend to be 'asian' in their heritage. ergo, most anime charicters tend to be white.
Anime in particular, it has more of a tendency to be based off of simple budget cuts. basic design teaches you that dark colors come up darker when surrounded by a lighter backdrop and often the dark brown colors of the traditional african decent individual will come up near black in the usually light blue and yellow backdrops commonly seen in most anime. and the only way to compensate for this would be to add better highlights and lowlights in order to bring out the detail.
because this takes extra time and money to do, most anime companies simply skip it and keep the cast 'white' or go about an alternate way to implement that person such as making them hav green skin...
Anime in particular, it has more of a tendency to be based off of simple budget cuts. basic design teaches you that dark colors come up darker when surrounded by a lighter backdrop and often the dark brown colors of the traditional african decent individual will come up near black in the usually light blue and yellow backdrops commonly seen in most anime. and the only way to compensate for this would be to add better highlights and lowlights in order to bring out the detail.
because this takes extra time and money to do, most anime companies simply skip it and keep the cast 'white' or go about an alternate way to implement that person such as making them hav green skin...