Retail was a progression game with new expansions coming every so often. By the end of WOTLK, even if you were dominating with the highest possible gear, it wouldn't matter because a blue item from the next expansion would be of greater value and rebalancing could happen between expansions now. This gave people joining in the game at the start of a new expansion the same chances to get to end game gear as people who progress from a previous one, that is, if they level efficiently enough. We don't have that luxury and catering to new players is just as important as catering to the old ones, otherwise, nothing good will happen in the long term.
Right now, excuse the use of the word, we are stuck on this expansion. It's our choice to do so, but we are sotuck non the less. People trying to progress can't progress because they are GY farmed by people at the end of progression and this is a major turn off for a lot.