Because the wiki was for an 18+ game.
Here's a picture of Furukawa's reaction to it.
ok Thank you, I dont think that it was a good Choice from them to delete this Walktrough since they Probably lost a Lot of Viewers.
They deleted the wiki.
And no, it was not a good choice of them. No one thinks so. And no one has made another wikia AFAIK.
EDIT: Furukawa's old walkthrough can now be found HERE