Because Legion isn't even remotely looking like WoD. It's too early to tell what it's looking like, but definitely not WoD. An emphasis so far has been made on dungeoning and world events with a minor side feature that resembles the garrison table and people kinda just freaked out about it. Do I want it in the game? Not really, but I also don't think stuff like that is bad if it's not made into a core feature of the game. As minor side-fluff, it's....still not great but not inexcusable.
I don't disagree that WoD is a trainwreck and for the time being I've jumped ship for about over a year now. I would call myself a Blizzard fan though because even though I don't exactly like the company as much as I used to, I still often play their games and enjoy them quite a lot and consider them to be one of the smartest and highest-quality game designers on the market. That being said, WoD was an easily avoidable catastrophe and watching it burn while there have been easy solutions right in their face (and in past expansions!) and watching it just go over their heads is mind-blowing.
That being said, I haven't totally lost faith in Blizzard yet because Mists of Pandaria was easily the best expansion they have ever made, at least in my eyes. If I write WoD and Cataclysm out of the equation, WoW has only gone up for me over the years. It's just that WoD was such a sharp and damning failure that I don't know if it can recover this time.