If you can recall, flying was disabled in WoD because it would break phasing as well as some quest and cinematic triggers. Remember the PTR servers where flight was enabled and how painfully obvious it was that WoD wasn't designed with flight in mind at all?
Player feedback - regarding how great MoP's Timeless Isle content was - simply gave Blizzard a convenient excuse to keep everyone grounded in order to cut corners and push the already-delayed expansion out sooner.
To add to this, Tanaan Jungle was so vastly incomplete at launch, that they placed an impenetrable dome over it and people still found ways of getting inside.
Given how time-poor the development team were at getting content out the door - as Legion's development was already underway - Blizzard had initially decided to disable flight for the entire expansion rather than test and fix everything that flying broke.
I'm tired of reading that pathfinder was introduced to reinvigorate world PvP and exploration, because it wasn't. That's the corporate spiel trying to cover up the fact that disabling flight substantially reduces the amount of testing that needs to be done prior to launch.
Any praise of the pathfinder achievement only emboldens Blizzard's decision to cut corners, which could partially explain why the game is where it's at today.
EDIT: This isn't a thread about pro / anti flight. Too many people repeat the rhetoric that Blizzard did this for the player's benefit, which isn't true, and I'm setting the record straight.