And there's some real money issues involved right now. WotC has stated that they want every supplement to hit that 100k sales mark. So far, they seem to have been hitting that target pretty well. And they are quite content to have huge lead up times to new books. I mean, Xanathar's has been in the works for almost two years - between all the UA articles on class and whatnot. That's unheard of for an RPG book.
There's also the point that there doesn't seem to be a huge demand for setting books, despite forum posts. None of those 20 settings are setting any sort of sales records. And, adding to that, if that 100k benchmark is true, virtually no setting book has ever reached those numbers. Sure, there are a few exceptions, but, they are very few and far between. Probably 99% of setting guides would be considered abject sales failures by the current benchmark.
So, if you want a new setting guide, start buying the ones that are already out there. Show WotC that there is demand for setting guides. THEN they'll give you what you want. Otherwise, it's just not going to happen. Why would they bother publishing a book that they know won't sell to the levels they want?
What I can see happening though is gradually opening up settings on DM's Guild in conjunction with new adventure paths. We are slowly seeing a bit of that with things like the Tortles and Ravenloft. Hopefully as new AP's come out, they'll loosen restrictions and you'll see 3rd party publishers picking up the slack on DM'sG.
But, IMO, the days of WotC banging out setting guides like we saw previously are long gone. It's just not going to happen. There's just not enough money in it.
