In the modern world, especially since Hitler etc , it became untenable to acknowledge the fact the human race contains / contained many geographically isolated and evolving groups.  And just like every other species, these groups became quite different, including their average intelligence level.  It's biology 101 , but much better for a modern world that seeks to assimilate everyone together, if we just pretend there's no average differences in anything of value to said modern world, eg, intelligence.