Simple. American society tends to value blond white girls more.
Notice you didn't question what the gender of the grandchild.
Maybe that occurred since the post specified “granddaughter“?
The post also specified the hair color
They questioned the hair color ("why blonde") but not the gender ("why a woman").
Seems to be clearly questioning the entire premise to me.
People are more sensitive to the pain of others when the victim is perceived as cute. See cute animals vs not so cute animals. We're instinctively wired to think that cute means innocent, helpless and deserving of help and protection. Obviously what is considered cute is subjective and culturally influenced to some extent.