The real reason is less about creating an actual “whites only country” and more about creating a political outgroup to scapegoat so they can stoke hatred on that line and cause the working class to be distracted fighting each other, so they don’t band together to fight their mutual enemy of those playing the puppetmaster. It’s been the goto strategy since the aftermath of Bacon’s Rebellion. Races don’t exist from a biological perspective, there’s more variation within a “race” than between them.