Because some of the crazy reasons people have had for believing in stupidities like the divine right of kings, blood purity, racial supremacy etc. DO NOT SURVIVE EXAMINATION when you understand the underlying process.

Once you re-name racial purity as, at best, a vulnerable monoculture (and at worst, inbreeding), racism becomes harder to defend intellectually. I have no idea how much its declining social acceptability is related to that. Probably not much. My intuition is that most "inbreeders" (as I call 'em) are not very intellectual about their racism anyway.