A lot of people aren’t interested in WoW these days because there isn’t anything that is really pulling them into it. WoW, as discussed by many content creators, has shifted its creative direction to the point that it is unrecognizable to what it used to be. This, clearly, was an attempt to try to diversify and bring in a new generation of players, but who they are appealing to aren’t going to stay or be as loyal as their long-term players.
In short, Blizzard is gambling very hard on WoW right now, and losing. Diablo IV is performing extraordinarily well due to its general theme, where as WoW is one side-step away from being as basic and simplistic in its storytelling and theme as a children’s Saturday morning cartoon.