Because there is more money in old content. They can literally roll out WotLK Classic now they have 3 Classic games. They have D2 remastered. Honestly they have a gold mine for years in old games. They don’t need a new WoW expansion.
Then they really do want D4 to succeed and it needs too in order to compete with PoE and regain their dominance there. This takes us into 2023.
Why not just work on the next project while all this is self sustaining making money. It’s the right business move. Furthermore at some point the “Last WoW Expansion” is something that will happen. There is no better time than now for that to be the case when they got years of old content to keep people happy while they work on new games.
There has to be discussions internally about SL being the last expansion. It makes sense then you can do 9.3 and 9.3.5 etc into 2024. This is just the reality of the situation and Blizzard will save money while making money they can funnel into the next big game/MMO