According to Blizzard, they felt that flying took out a lot of fun from the game, and it allowed players to escape combat or avoid combat entirely, an experience that the developer would prefer if players partook in. According to a post from a World of Warcraft community manager, “flying trivializes combat… Flying allows you to escape or enter combat at-will. There’s a reason why flying isn’t allowed in dungeons and raids, or battlegrounds and arenas, and that’s because it would trivialize the core mechanic of the game in those areas: combat. For much the same reason it trivializes how content is approached in the outdoor world based on the simple fact that you can lift off and set down wherever you like.”
Flying will be introduced but like we said, only in patch 6.1. It will also not be as simple as players forking out gold to gain the ability, but rather something gamers will have to work towards unlocking, which we can only assume would involve some kind of quest line. This is similar to how back in the days of vanilla World of Warcraft where Warlocks had to undergo a lengthy and expensive quest chain to gain their mount, as opposed to today where players just have to pay a few gold at the trainer to get it.