Talavaj already explained the reason for this. I`ll just add that warlock zoo decks are your best bet to rank early on and to gain gold quickly. They provide quick games and you`ll win a lot.
The Zoo deck (depending on how you build yours) should only cost around 1500 dust, which is less then one legendary card (which many decks require a few).
This is what frustrates me about the game, but is probably unavoidable to some degree. True, Hearthstone is simpler than most deck based games, but it`s also become so popular and widely played that the most efficient decks will rise to the top much faster than a game that requires you to meet up with other locals to play. It`s a flaw in the simple fact that it`s an online game.
Oh well, I still love it. Here`s hoping the expansions provide more depth to the game.
You can't blame that on Warlock being OP. Zoo is just a very efficient deck. It uses the most solid cards in a deck. Obviously it has a major win ratio, that's why it so popular and the skill requirement is so low any new player can make it and be good with it.
But this doesn't make zoo only playable in Warlock. Paladin/Hunter Rush/Zoo works equally well, even without the extra card draw. Obviously Warlock is top tier Zoo because of the ability to throw everything on the table early on and then play Soulfire and Doomguard without ANY drawback. But there are pros and cons to both
Zoo is so damn good because its so easy to make, costs almost as little as 1200 dust without legendaries. But here is where Zoo shines, you don't need expensive cards to win. You just a good deck build and a simple strategy:
Trade Well and Go for the face!
Good reddit discussion about zoo:
http://www.reddit.com/r/hearthstone/comments/20na9h/what_makes_a_zoo_deck_and_zoo_deck/
Zoo isn't new. Basically the concept first appeared in Magic the Gathering. You toss all animals in there who are low-cost, efficient from the get go and pop some finishers late game and you have a "Zoo"
Hearthstone and other TCG/CCG adopted this concept and made similar decks to match the original Zoo concept.
Magic The Gathering Zoo Deck & Explanation:
http://mtgsalvation.gamepedia.com/Modern_Zoo_deck
Back to the point, Warlock isn't OP. It just works really really well with the Zoo deck, because you can empty your hand till turn 4 and then finish them up with Soulfire and Doomguard that can be played FOR FREE. Also, once you start topdecking, you draw two cards instead of one, always giving you more options and quickly fishing for answers.
One of the best if not THE BEST Zoo deck (Have been using it since the first edition)
http://www.hearthpwn.com/decks/35326-budget-reynads-zoo-v2-warlock-aggro
Instead of crying about it, you can make Zoo and play on ranked. See where and against what you lose.
I can tell you for a fact that every class can easily beat Zoo, but Zoo can also overwhelm any class. That's why the tradeoff is worth it.
Basically cheap area effect cards go well against zoo or if you get to the late game, the late game area of effect spells.
Any combo creatures that allow for early game board wipe also do well.
LIST OF ANTI-ZOO CARDS FOR EVERY DECK:
BITCHSLAP! Seriously, just add 2-3 of these in your deck and it will already be better against zoo.
Of course, THE BEST combo against all zoo/rush/tempo decks is:
+
