Additionally, who cares about eSports in WoW? It's an RPG; an RPG will always inherently be less balanced and less demanding on player skill than other options. If you want to flex your e-peen and talk about skill, go play a game like Brood War, Quake, or Street Fighter. The point is that random BGs - the type of gameplay that the vast majority of players experience, and the ONLY type for many of those - are exceptionally frustrating due to 3v3 arena design and balance decisions not scaling well to the 10v10 or 15v15 format. Designing and balancing around rated BGs would eliminate a considerable portion of this frustration simply because the only meaningful difference between a random BG and a rated BG is teamwork and coordination.
And the issue with a game like WoW is that design choices made for a duels-in-a-box environment like arena simply don't scale very well at all to RBGs. Melee and ranged classes are equally effective in arenas, where maps are small and there are many LOS objects for melee to use to avoid getting stuck in the open, but this doesn't apply to BGs on the whole - maps are enormous, and while there may be some LOS objects around key objectives, the majority of the map is wide open, which is why ranged classes dominate while melee classes are weaker to some degree.
Additionally, dying in arena is extremely bad - you only get one life, so if you die, that can easily mean game over if your surviving teammates can't very quickly kill an enemy player to compensate. In RBGs, it's not quite as bad - dying is still a bad thing, and it can still mean a lost game, but the impact is ultimately lessened.
Moreover, because you have larger teams, classes do not need to be (and I hate this word) homogenized. You don't need every class to have self-heals, or a disable, or an interrupt, or whatever; you can build a team to cover any weaknesses a teammate might have, while the enemy team can build a team to try and take advantage of any weakness. Essentially you can look back at vanilla, before arena ever became a thing, as a broad-strokes idea of how that works; people still played BGs competitively for rewards, and even though not every class had an interrupt, a disable, self-healing, etc, it still worked. It can still work again, but it would require a shift away from arena and a shift towards rated BGs as the sole design goal.
The entire point is that, of all players who engage in PvP at all (which is already a small number and has been getting smaller over time; 5.3 may slow the bleed but I don't expect it will stop it entirely), random Battlegrounds are almost always the first and largest amount of PvP they do. Because of the design focus on arena, this results in an exceptionally frustrating experience on top of the required gear grind (which is thankfully being lessened with 5.3), resulting in a LOT of players giving it a try, going "Wow this isn't fun at all," and never touching it again.
Since Blizzard is still investing resources into balancing and designing PvP, this is bad - it means there's an entire section of the game they're working on that many players won't play because it simply isn't fun. That's bad for business. From a purely financial standpoint, it makes sense to design and balance around the playstyle the most players engage in, and particularly the playstyle the "casual" (another word I hate) audience prefers, since they make up the lion's share of the playerbase.
Balancing around random BGs would be stupid. But you can balance around rated BGs, and it results in an almost direct translation to random BGs because - again - the major difference between a random BG and a rated BG is just teamwork and coordination, not an entirely different gameplay style.