WoW has been there since the beginnings of streaming. When twitch started to really go big, so did WoW. And WoW had arena to stream. So thats one of the reasons why it still stays there.

MMOs are generally not very intresting to watch. Specially not anymore.

You need huge personality to get lot of people intrested watching people afk in Eulmore for 20 mins to get into dungeon roulette (and no one wants to watch Satasha so they skip leveling roulettes for that reason, lets be real). Sure they could heal the dungeon but who wants to see someone spam holy for 20 mins?

People want constant action. The more action the less streamer has to keep people entertained. And no im not saying it just like "kids these days with their snapit tikitoks".

MANY genres suffer from this. It has to be easy to understand as someone who has NO idea about the game.