Because whites see blondes as being pure not many people (their are some) have blonde hair naturally that aren't white. It represents pure whiteness to some of them.
It's stupid logic as whites can have all types of hair but I think that's why a lot of them hype it up.
blonde hair is a recessive trait as well which means the only way for people to get it is to be very very

like as in on an extreme generational level. a group of people can never become that way without a lot of that going on because dark hair is very dominant
thats why you dont see blonde bi racials like that. blondeness is a sign of pure, almost absolute, whiteness.
but the white people with dark hair and dark eyes i dont get that same vibe from as far as that aspect is concerned. they dont self hate for having dark hair and dark eyes. only asians and blacks to me really seem to self hate in that way. the dark featured whites seem to actually love their dark features. arabs give me that impression as well. they dont put that absolute whiteness on a pedestal. i think alot of them see it as inferior to them. its an interesting example because its a proof in my opinion that absolute white beauty standards dont have to dominate and in reality a lot of that is conditioning and installing a sense of shame and some aspect of their culture allows them to avoid that and retain pride in their features.