I think its because of the decline of MMOs as a genre.
It used to be that you needed an MMO to meet players and group up / match with them. All other game genres caught up. They have matchmaking and grouping now. The days of walking around and socializing to meet people are gone, folks want quick matched content, MMOs are not very good at that (they’ve tried to adapt). Essentially, MMOs are like the old “playstation home” or “second life”. Folks thought an open world to find people was what the crowd wanted. In reality, thats not what most people wanted.
Some kind of innovation in the MMO genre is necessary to make it relevant again. Until then, WoW has enough momentum to just keep surviving.