> Cambodia is blatantly racist
In my experience, there is nothing special about Cambodia in this regard. You could substitute any poorly developed nation, and you would have similar results. If Cambodia had a GDP per capita similar to Japan/Korea/Taiwan, they would "suddenly" become less racist because they would be much more concerned with economics rather than ethnicity/religion.
It's human nature to be "racist" since people generally prefer to be with others similar to themselves. That can be multi-dimensional, ranging from values, cultures, tastes, but race is the most obvious and one of the major principal components of human experience.
Japan, Korea, and Taiwan are effectively ethnostates and they've not had the cultural enrichment yet that America or Europe has faced. Notably, there is rising anti-migrant extremism in the West, because the Great Replacement Theory is becoming increasingly obviously not just a theory.