Because the massively awesome Wrath Gate cinematic and the entire cool stuff thereafter only unlocks after completing a whole bunch of mini quest chains to get cheap wow gold.
If you are Horde, you will need to do the quests in Westwind Refugee Camp, Venomspite, and Agmars Hammer. If you are Alliance, you will need to do the quest in Wintergrarde Keep and Wintergarde Mine. Once you have completed the pre-requisites, the faction leaders in the zone (who, incidentally, are also duking it out in the Isle of Conquest) will send you off to see dragons, treacherous undead, and flashy moves by your factions big shots to get cheap wow gold.
At the end of the day, you will come away with some nice loot such as flashy pants -- Wrynns Legplates of Carnage or the Warchiefs Legplates of Carnage (healing Paladins will need to pick up the spell power Mail) -- and a nifty trinket. Completing the Wrath Gate sequence will also irrevocably phase you into a different version of Azeroth where some NPCs are no longer around. Think about this whole phasing deal for now, because it looks like it will be pretty exciting in the near future about wow gold. Did I mention you should level up mostly in Dragonblight for this period?
Warlocks will re-roll en mass, finally discovering that felhunters are ugly and difficult to housebreak, and having them totally does not make up for the sad fact that Warlocks are not Mages about wow gold. Also, this will only happen in my imagination, where Warlocks do things like this all the time.