What did we do wrong?
IMO - they copied WOW too closely, took too long, and bolted on a wonderous, immersive, vastly expensive
fully-voiced story mode that people enjoyed and played through - and then quit once the story came to an end. Which shouldn't have surprised anyone.
Never give people what they say they want - by the time you've made it, they want something else.
And that's from someone who actually likes the game and has subbed since day 1,.