The simple answer is because their livelihood requires making content that people actually want to pay for. Japanese people in particular, who I can't imagine even give a shit about introducing muh diversity. I don't think the activists would be very happy with mangaka making a story of blacks anyway, either because it would be way too realistic for them (e.g. a gang of murderous thugs) or muh cultural appropriation.
The simple answer is because their livelihood requires making content that people actually want to pay for. Japanese people in particular, who I can't imagine even give a shit about introducing muh diversity. I don't think the activists would be very happy with mangaka making a story of blacks anyway, either because it would be way too realistic for them (e.g. a gang of murderous thugs) or muh cultural appropriation.