Honestly? Why publish new settings? The existing TSR offerings cover a pretty hefty chunk of the spectrum! I mean, you could argue there may be a niche or two here and there that could still be filled. Eberron was pretty precisely calibrated to fill the need for a 'magitec' setting, which is really the biggest thing missing from TSR's offerings (Spelljammer is KINDA magitec, but not really). I think WotC sees settings as more of an adjunct to other parts of their product line, frankly. They're willing to put out something like an M:tG inspired setting for instance, but that seems calculated to increase sales of both 5e and M:tG products. I doubt they believe it will make money on its own!