I think it is lack of resources (and will). Heroes of the Storm had skins that would change the actual appearance of your kit (as well as the sound FX and visual FX, they put more work in the HotS skins than they did the WoW customization)
“But Microsoft owns Blizzard now, they should have unlimited everything!”
The gaming industry ain’t what it used to be.
Also I think we are all underestimating how old the WoW engine is. I wouldn’t be surprised if they don’t add anything because the engine can’t handle it.
I’ve done everything I can possibly do to improve my performance in Dorn and I still drop to 40-50 FPS. The only thing I haven’t done is disable addons completely and upgrade my CPU.