Because they could tack the wow sub into game pass to justify raising the game pass price and then charge the millions and millions of people more game pass money that don’t have anything to do with blizzard games.
The development costs would more than likely be the only reason it won’t happen. How many years would it take along with how many people to do what you want of putting this game on an Xbox how many said people would play this version worldwide. It would have to have a very good cost benefit ratio otherwise it won’t happen. They are after all in business to make money for investors not us.