Ok, it is more than a little ironic that your disputing my poll (I'll admit it's flawed) when you're making pretty authoritative statements with no evidence at all.
There is only one piece of solid evidence, this monthly survey done by D&D itself, which polled setting popularity.
The popularity of settings in the survey fell into three distinct clusters. Not surprisingly, our most popular settings from prior editions landed at the top of the rankings, with Eberron, Ravenloft, Dark Sun, Planescape, and the Forgotten Realms all proving equally popular. Greyhawk, Dragonlance, and Spelljammer all shared a similar level of second-tier popularity, followed by a fairly steep drop-off to the rest of the settings. My sense is that Spelljammer has often lagged behind the broad popularity of other settings, falling into love-it-or-hate-it status depending on personal tastes. Greyhawk and Dragonlance hew fairly close to the assumptions we used in creating the fifth edition rulebooks, making them much easier to run with material from past editions. Of the top five settings, four require significant new material to function and the fifth is by far our most popular world.
As you can see, FR, Eberron, Ravenloft, Dark Sun, and Planescape are all EQUALLY popular. Of these, FR and Eberron have a setting book, and Ravenloft has Curse of Strahd which is not-quite a setting but a good guide to a piece of Ravenloft (Barovia). Dark Sun has literally nothing, and Planescape has an adventure to 1/9th of one plane (Avernus).
If we were to rank the level of content of these five equally popular settings, Forgotten Realms has by far the most so far. Eberron has a solid setting book, but no other content except from what is found on the DMsGuild. Ravenloft has Curse of Strahd, but FR of course has ELEVEN published adventures. Planescape has one adventure, which it actually shares with FR, and Dark Sun has squat.
If we were to rank who should get a setting book, it would thus be;
1. Dark Sun
2. Planescape
3. Ravenloft
4. Forgotten Realms
I actually find that a pretty fair release schedule.