They wanted it to be a live service, which is famously expensive to develop. They have to pay people to be constantly developing content on top of the insane prices of releasing and marketing a AAA game in the first place. Usually these games make money through microtransactions and/or subscription fees but no one wanted to play the game in the first place so hardly anyone was buying any of the extra content after initial release.
That’s the most baffling MMO decision I’ve seen, tbh. WoW has plenty of issues but at least they aren’t just deleting the continent of Northrend to save on install space or anything