By breaking any semblance of balance it had with saying that "we balance around 3v3". And then it turns out 3v3 is even less balanced than before.
In WotLK S6-8 2v2 was pretty well balanced. Hell, even 1v1 didn't have the hard counters like today. WotLK did A LOT of things right. From PvP balance to arena rewards to raids. There's a reason they saw such a huge jump in subs then and a loss of subs in Cataclysm.