This was the question at Battle of Azeroth Media Day to which the developers apparently don’t have an answer yet. The only reaction from gaming community to this bit of interview was: REALLY?!
And I mean it: really? After all these years of game development and received feedback Blizzard doesn’t have an answer. The only reason they gave is that players feel stress knowing that there is a real person behind the opposing character. That’s the most ridiculous reason ever to my opinion.
Let me clarify the mystery from the point of view of a PvE-only player.
First of all, here’s my achievement chart to give a perspective at the author of this blog:

As you can see, PvP is the least developed – and thus the least interesting activity that I go into during my playing time. The reasons are as follows.
Competition Is Not King
People play for many reasons, and one of them is competition of course. Even in PvE pushing the limits is sometimes vital: be it raid progression, clearing mythic dungeons, soloing current raids and other stuff.
Still, PvP is ALL about being better than the others, ‘kicking their ass’, overcoming the real players. Dear Blizzard, could you imagine that there are people that are NOT interested in this?
I’m not interested in competition – I login to see the story, to explore the wonders of the world of Azeroth, to relax and have fun. I’m not interested in pushing my buttons better than another person – like at all.
Immersement
PvE is not free from the following drawback, but the amount of nicknames like ‘Bobpro’, ‘Kickyerass’ or ‘Shamanheal’ is significantly a more frequent thing for PvP-oriented people. And these nicknames are a litmus test, they mean that the player behind it doesn’t give a shit about the world he/she’s in. Their character for them is just a sack of pixels and stats, there’s no backstory, imagination or immersement to the world involved.
Needless to say, this ruins my immersement as well. While any book, movie or game is a sort of escapism, you want to spend your time in a wholesome world. Or at least try to.
When questing, world questing or farming, you have your own goals to achieve. An unexpected PvP encounter (even if you win) is a disturbance, it stalls your progress towards your goals. It is not wanted, it is not welcomed, and well, it is not fun.
When I’m in the middle of an exciting story, I don’t want to be interrupted by a random ‘Bobpro’ who decides it’s time for PvP for him.
Social Stigma
PvP, in general, is shaking the virtual penis in front of the fresh corpse, knowing that there’s a real person behind it.
It is obvious that this kind of activity attracts lots of total jerks who just find it a mere pleasure to kill other players. Are they compensating the real life failures and lack of achievements (a thing not uncommon with younger people), are they just angry at the world and want to punish a random player for it, or just your common griefers whose idea of fun is to ruin the other person’s gameplay.
World PvP modes allow them to do this. When you’re in the middle of a quest, fighting a harder zone boss ,or 3 mobs at once, a guy comes at you to finish the job the mobs started.
Most often, he will also jump around a corpse. Nice job indeed!
Cherry on top, the guys won’t do it for the Alliance or the Horde’s sake which would at least justify the attack. It’ll be your random ‘Bobpro’.
Surprise Attacks
World PvP in most cases is a rape. The aggressor starts the fight having a huge advantage of being prepared, having his cooldowns at the ready, and the element of surprise. This alone is enough to burst-cut the foe to 30% before he even thinks of defence.
And there is more – class imbalance. A number of classes, especially caster ones, need time to fuel up and start to deal some notable damage. Not to say anything about healing specs with the lack of offensive abilities.
Basically, an equally geared warlock has no chances against a warrior or a death knight surprise attack.
Ganking
What else to say here? Only that scaling world puts an extra 15-45 level gap between players which don’t scale.
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When Blizzard asks why some players are not into world pvp, the list is welcome to be quoted.