Its the lack of difficulty.
People try to dive deep and analyze why WoW is sinking and point to all sorts of things like Azerite, LFR, CRZ, Sharding etc. Those things play a role no doubt.
But from a design perspective the real issue is the overall lack of difficulty across 95% of all content. Blizzard has to spend tremendous amounts of work hours, talent and money to create content patches, but because of the low difficulty of all this content the playerbase consumes it almost immediately and is left wanting more. This obviously isn't sustainable, yet this has been the Blizzard approach for many years now.
Contrast this with the design approach from Vanilla/TBC. A single raid could easily last for 6 months. Just a raid, not even any other content to go with it. Not even multiple modes of the raid, just purely one single raid. People would be out in the world farming various gear, items, enchants, oils etc for this one raid, and they needed that stuff because the difficulty of the raid necessitated it. And the process of getting those things was challenging itself. Gold, mats, enchants, everything took time and effort to get. The time and effort kept the playerbase busy playing the game rather than complaining about it.
Further, the lack of absurd catchup mechanics meant that old raids were still viable content even after new ones came out. Players continued raiding SSC and TK for months after Black Temple came out in TBC. In short, the content Blizzard created got used to its full potential, lasting months or even years.
Its funny, the logic behind creating easy raids was that since they were so expensive to develop that more of the playerbase should see them. They were partially successful in their goal - much more of the playerbase sees raids now, but they failed in that this expensive content is an even worse investment now due to how quickly it is consumed.
Classic will be an interesting case study to compare side by side with BFA. The slow methodical reward structure of Classic vs loot falling from the sky in BFA for doing next to nothing. Hopefully it will show Blizzard there is nothing wrong with actually making players play the content that they spent months of time and millions of dollars creating.