Because it exists. There is a vocal, toxic minority in this game who seem to believe that flying is bad for the game and want it to be removed for everyone. The key points to their argument are:
1) It kills WPvP
2) It ruins immersion
They believe that this justifies forcing their demands on everyone
That's a gross oversimplification.
I think there are a lot of people who care about flying but were quite content to wait. We generally saw the merit in the arguments for witholding flying from new content and were generally content with the way WoD was handling it, operating under the flawed assumption that at some point flying would be introduced. And so we didn't really say much on the topic.
What we didn't count on was how the anti-flying camp was actively working to try and push Blizzard to keep Draenor flight free. Hence the "losing our shit" moment when Blizzard made that particular announcement.
And no, I don't think it was really a case of "losing our shit". It was a case of waking up and realising that our silence on the issue was to be taken as tacit approval of the crap being fed to Blizzard by the anti-flying crowd. A lot of people felt that the decision to not introduce flying was a bad one, and made their opinion known.
So much so that Blizzard quickly changed their minds. I honestly don't think Blizzard had any idea of how the majority of players actually felt until we spoke up.
My take on it is that the anti-flyer crowd are butthurt over losing this one and are continually asking these questions in the hope for some vindication.
And no, it's not that flying is a feature that is "amazingly fun" for most of us, any more than getting into your car to drive to work is, or the ability to flick a switch in your house and have light, or running water, or toilets or any of the other myriad mundane things we have that make our lives easier. Sometimes you need to take something away to realise the impact it makes in your life.