Because this is an MMO and it's supposed to move forward.
Because WOTLK also got old after a while, oh yes! People complained about it lasting FOR TOO LONG! Just like SoO and DS.
Because they want to work on the game moving forward, not spend resources and money feeding nostalgia junkies
Because every single nostalgia junkie wants a different part of each expansion, since they will also have a patch that they hail as the pinnacle of gaming.
There's plenty of hardcore content right now, hardcore players are busy playing though instead of bitching on forums about wanting to go back in time. Convenient huh that you can't be hardcore unless in a different expansion than the current one? "Because current content ain't as hardcore!!". Lol!
And so on.
It.Won't.Happen. People wanting stagnation, can keep to single-player titles.