Right. But WotC has decided to limit the number of books they release annually, as part of their overall "don't flood the market/overwhelm casual players" approach for 5e. And that means they want every one of their limited number of releases to sell as well or better than whatever else they might have released instead in that "slot." This is why, when SCAG sold less well than any other early 5e release, they put the kibosh on further setting guides for a while.
Then, when the corporate synergy of a Magic: The Gathering setting proved irresistable, they did Ravnica, and it sold extremely well, opening back up the setting guide option for official releases.
However, they've now done three consecutive setting guides (Eberron, Wildemount, Theros) and even though those three all seem to be selling/preselling very well, I'd be surprised if we get yet another setting book anytime soon.