Wasn't there some old WD Legendary changed so that the poison darts piece? Pretty sure there were at least 2-3 Legendaries changed for WD.
Blizzard has said many times that they plan on fixing Wall of Zombies before changing the Helltooth set. I am thinking some big rework is planned for that. Until then they are basically just focusing on getting Monks up to par hence why Monks have been changed the most in the last few patches.
Yeah, there is a dagger that makes poison darts pierce and they are also retooling The Gidbin to make it less terrible, not to mention they are fixing fetish spawn deaths which will make Zunimassa less terrible (though I'm not sure if this will make anyone use it)
Honestly it shouldn't be a surprise other classes got more legendary changes than WD, they were already in one of the best places item wise. Jade set, matching helmet, SMK, Mask of Jeram, Tall Man's Finger, Thing of the Deep, Uhkapian Serpent. All of those are top tier items. Witch Doctor's currently have more T6 capable legendary items than some classes have legendary items total (looking at you barbarian).
I mean that's not to say there isn't work to be done, but, right now there was higher priorities which I feel like blizzard has basically said in the past when they said they wanted to use WD as a model for items for other classes.