The thing responsible for the big success, is also responsible for its slow demise.
With every big content patch or with every addon you are starting from anew. Class-changes and redesign made the "rinse and repeat" very attractive. But at some point people are not interested anymore to follow this "rinse and repeat" stuff after years. At some point you don't see the lore anymore or any other visual distractions, but only the "rinse and repeat" mechanic behind the game.
new colours, new mechanics, new items, new models, the lore is going further.... but at the end the game experience / the pattern follows still the exact same way like a decade before.