Hearthpwn

Even at 4 mana might cost you a full turn doing nothing to play it, which usually means death against aggro decks. Deck of Lunacy is not played for this reason any more. Like Lunacy Deck of Chaos does not buff your hand, you need to draw your advantage, which might be too slow after turn 4-6 (which already requires a lot of luck).

Deck of Chaos is still a meme card. Even if you get the buff early, you run out of steam against control decks, like infinite board removal mill warlocks or endless value generating priests. Against aggro decks you usually just die before getting value from DoC.