Because Blizzard's class identity for rogue includes good single target removal and bad aoe removal. They don't want it to be good.Yeah, but it was double nerfed. It lost the ability to do damage to face, but it also got doubled in mana cost. It's so terrible at removal that many people thought that even Envenom Weapon couldn't make Blade Flurry playable at 4 mana. Why is it so extremely expensive?