Because the early Sylv story was about how Arthas killed and controlled her, which fits in with the same type of trauma that those things can leave on a person. I think a lot of people were reading her story for a while as a survivor coming to grips with what happened to her, and then that story was thrown away to just have made her a over the top evil doer.
I’d also note I think many times when this comes up it can be in response to people suggesting a redemption arc for Arthas, especially one where he steps back into that role with her, and that would be a really horrid turn in the story.