Uses up more ink? Banding issues when encoding? Naruto was switched to using that silly headband, because the artist didn't want to have to keep drawing the original goggles.
Comparing the few number of dark skinned people in Japan, think they are well represented in anime. Can't expect them to be shown in a typical school setting, which most anime is revolved around. Well, not counting Ganguro Girls. Can watch Super Gals if want more of that.
Skin lightening cosmetics is pretty popular with Asians, made by popular western companies, which would be taboo to widely market here.