Because those who left Africa, with high levels of melanin, became gradually white, as a consequence of NOT having to fend off blazing, burning, scorching sun.
When they met their fellow brethren they thought themselves sophisticated, urban, and frankly too well endowed in very sense that mattered, to be related. Hence they took the initiative and demoted all those not "white", to proclaim themselves up the hierarchical pecking order.
"Cheddar Man: Discovery first modern Briton had dark skin is reminder 'we are all from Africa', expert says
'It just challenges this idea we have that certain people belong to certain places – that Britons are Britons are Britons'"