Why did World of Warcraft fall out of eSports?
Inconsistent Patch Dates
It was truly difficult for any top 3s team to defeat RMP except in one particular patch which buffed beast mastery hunters and enhance shamans. This paved the way for the legendary "Beast Cleave" 3s composition. Only 3 days after the patch, all the top arena rankings on all battlegrounds had Beast Mastery Hunter, Holy Paladin, Enhance Shaman. The forums were flooded with "please fix burst damage on enhance shamans and beast mastery hunters" -- Every arena match you would hear Blood Lust (ARARRGHGHH), the spirit wolves would come out and the Beast Mastery Hunter would pop BM -- and your mage would be dead in a global before he could even Ice Block. Incidentally, one could say most of the patches that came out were really to address PvE issues -- to help diversify raid compositions (bring in more enhance shamans instead of stacking rogues or fury warriors as DPS). Blizzard did a great job of addressing these issues and I agree with them for taking the right approach -- to focus on the larger set of users on World of Warcraft -- which happened to be the raiders.
The eventuality of World of Warcraft in eSports
With MLG, IEM, and ESL dropping support for World of Warcraft around 2010 due to game balance issues, inconsistent patch times, and watchability issues -- naturally, PvP became less popular with World of Warcraft's user-base. Looking at the lessons learned, in order to make a game truly eSports friendly, there are two core aspects that really make it enjoyable for audiences: watchability (how easy is it to understand what is going on) and game balance. Naturally, MOBAs like League of Legends, DotA, and soon Heroes of the Swarm are easy to watch and I definitely see these games growing in viewership in these games in the coming years.
The future of competitive gaming is dependent on the viewership and the competitive spirit behind eSports. Riot Games has done a fantastic job of making League of Legends what it is today -- the most popular eSports game in history (although, you could say Starcraft was pretty big too). Ultimately, I'm sure we'll be seeing a lot from Blizzard Entertainment when Heroes of the Storm launches, as well as, the launch of Hearthstone on mobile devices.