Going out on a limb here but I think it's because of the portrayal of the house elves. A subjugated people who everyone said was better off in slavery, and the notion of emancipating them is ridiculed. Some people write it off as a coincidence, but there sure is a lot of these coincidences, like how the entire wizarding world's economy is run by hook-nosed evil creatures, that even conspired with the dark lord for their own gains.
In a since-deleted post on her site Pottermore, the writer argued that while freeing the slaves was noble in isolation, being enslaved is what's better for the elves right now and they should stay that way until they politely ask to not be enslaved.
I don't know who the original writer is and I don't know of an archived copy before Pottermore ended and everything was transferred to Wizarding World, but I believe it's from an era of Pottermore where she wrote the blog entries herself
Nah the actual in universe explanation is that Dobby is weird for wanting to be freed. Literal direct quote from Goblet of Fire
“But Harry set Dobby free, and he was over the moon about it!” said Hermione. “And we heard he’s asking for wages now!”
“Yeah, well, yeh get weirdos in every breed. I’m not sayin’ there isn’t the odd elf who’d take freedom, but yeh’ll never persuade most of ’em ter do it — no, nothin’ doin’, Hermione.”
Hermione looked very cross indeed and stuffed her box of badges back into her cloak pocket.
Still no, it's still a fictional story, just because you write a slave character doesn't mean you're pro slavery, just because you write a misogynist doesn't make you a misogynist, etc.
Again, she still might be, but citing Dobby as your evidence is stupid.