Alright, allow me a bit of horrorsperging.
The appeal of Lovecraft and Rowling (I like reading both) is entirely different. Lovecraft wrote what I would call "absolute horror". There is no redemption, safety, protection whatever in his tales. The narrator is usually about to die or go mad by the end, or already has done. You generally feel sorry for him but only in the sense that you wouldn't want to be in his shoes, there is not much empathy. This is the literary equivalent of a shot of absinthe, you're just left thinking "oh wow oh fuck". You don't need to like the guy who writes this and in fact you can't imagine someone who is not a sick puppy writing this stuff. Regarding the racism specifically, while he describes Blacks and Asians/Australasians in toe-curling terms (I'm aware very few Farmers care about this obv.) even Italians and Poles in New England are described as superstitious primitives and e.g. in The Dunwich Horror the inhabitants of Dunwich are inbred, incestuous and degenerated descendants of WASP settlers who nearly cause the end of the world. The guy just really didn't like anybody, people repulsed him, life repulsed him. It's part of the package. If you found out Lovecraft once kicked a puppy, you'd just shrug and think "yup, that figures".
Obviously Rowling offers the polar opposite of that. While her HP books are madly more violent and disturbing than people that haven't read them usually suspect, they are comforting and deeply moral. Characters on the page are practically "turning to camera" to deliver lessons, e.g. lying is generally a bad idea, you have to stand up for yourself even against your friends, don't let fear consume you etc... Obviously she is being completely true to these lessons, but this now clashes with the quasi-religious view of the world TRAs have built. No wonder it breaks their brain to find her standing against them. It means they are the bad guys and they can't accept that! Hence the frantic efforts to burn down the whole thing by hallucinating racism and other flaws in the books.