The problem with MMO writing is the player. Having to write around the player so the player is the one doing things is what sucks the ability to act out of the numerous central NPCs.
Example: Horde side Uldir, Talanji sees Zul reborn, yells about stabbing him, doesn't do shit, cus players.
The only time characters do actual things is during some cutscenes and in the novels where the player is absent.
Then there's the patch cycle. Months go past of characters doing nothing before the next content patch drops and some select number of characters do something in the plot, but not too much or too many characters because screen-time is limited so as not to take the spotlight off the players.
This is why Wacraft III was a good story and WoW has never been.