The problem is that this is a straight up ramp deck, and when you play a spell that takes up your whole turn after multiple turns playing shitters or rampant growth effects, you need to be getting more out of it that Malf gives you.I will not have you insult my good rotten seed Malfurion. If you can get a fandrel, then you can fill your board with annoyances. Not only that but his hero power is great for board removal and health denial.
Primordial Drake, Infestation, Lich King - these cards pay you off immediately and help you get the board under control. Malf doesn't and is very bad at stabilizing. It's obviously a very good card but maybe not here, maybe later if other stuff gets nerfed.