The guys over at amped esports recently published an interview with Blizzard Senior Community Manager Paul Della Bitta. Go check it out...
How did WoW become an eSport?
With the introduction of the arena system in the Burning Crusade, we realized with the ladders and the rankings online, we had the basic platform to start running World of Warcraft ladders. So, we decided to start our own tournament this year to see if the community was interested and see how it played out as an eSport.
http://www.ampedesports.com/news/3573/B ... lla-Bitta/