Thats like asking why white people in north America and Australia aren't dark like the native americans or abos.
The people that currently reside in the places that theyre in weren't always there. They either migrated there in the prehistoric past or in the recent past.
Most of the times with modern humans, it was the lighter skinned people from the north that conquered the darker skinned people to their south and mixed with them (cucked the darker skinned men they conquered).
But in some occasions, it was small groups of lighter skinned people seeking refuge in southern regions (not conquering them but seeking refuge) that get cucked by the darker skinned men. Such as was the case with the early Austronesian farmers from Southeast China/Taiwan who went as far as Madagascar/Polynesia and got cucked by the Bantu males and Polynesian/Negrito males. Or the stateless Miao-Yao tribes from central China who ventured southwards into southeast asia.
Or the primordial Chinese hill tribes to the very west of China in the Nepal/Bhutan/Bangladesh area that got cucked by curries. Or in very recent times with the Rohinga situation in Burma, the curries trying to cuck the lighter skinned Burmese but the Burmese weren't gonna have it and started raping/massacring the curries. That Rohinga episode very recently in Burma, highlights and showcases basically what human pre-history was all about.