The Warlock Zoo deck is a combination of three elements. First are the efficient minions to claim the board. Second are the buffs which enable trades. Finally is the Warlock Hero Power to keep the engine running.
The Zoo concept that will never be completely out of fashion. Playing more cards than your opponent always creates opportunities and the Warlock Hero Power is ideal to support such a game plan. Zoo decks existed in Hearthstone already before the first expansion – and while the Shieldbearers, Shattered Sun Clerics, and Harvest Golems were replaced – the archetype itself is still alive and kicking.
The fortunes of Zoo have varied, but it has always been present. Right now, Zoo is living its golden age, and is one of the strongest decks for many players. Both Midrange Druid and Secret Paladin are weak to an aggressive board control strategy. Given the popularity of these decks there is plenty of prey for Zoo to hunt. The Druid population also keeps the greatest nemesis of Zoo, Freeze Mage, at bay.