One, every other top game I can think of is linear, and so a walkthrough makes sense. The quests in HI are linear, but the game overall is not.
Two, when the walkthrough was posted, it did not reduce the number of common questions to any appreciable degree, even at first.
FWIW, I've argued for a separate FAQ thread, which could have a post for every common question (and then add posts with each update); to this point, there has been no support for the idea from newer posters, who would be the main ones to use it, and so no one else thinks it's worth doing because no one else believes it would stop the constant flow of repeated questions in this thread. (Tell truth, I don't think it would stop the flow either; but a) I do think it would significantly decrease the number of repeated questions, and b) it would enable all such questions to be answered with a single link to the relevant post in the FAQ thread.)