SCAG is also a big seller after nearly five years in print: it's not been poorly received.
I didn't say the SCAG was poorly recieved?
That's not a really true comparison, as the Sword Coast is much smaller than Khorvaire and Wildemount. It would be like having a whole book on Breland and the relegating everything else, including the other nations in Khorvaire, to a sentence or two. Heck, Xen'drick in the 5e Eberron book received probably 10x the coverage than Cormyr did in SCAG, and it's an entirely different, mysterious continent, not a well-known, next-door land.
Others have responded to you on the scale point, but I'll respond to the "But Eberron's book is longer" point.
Eberron's book is longer because it adds a slew of new races (and re-adds others like orcs/gobbos), an entirely new class, and an adventure. It is not double the amount of "gazzetteer material" covering the areas/lands.
Blah blah blah. Read the OP.

and again I will ask... why do people keep assuming an FR book means covering everything like Kara Tur and Al Qadim or going into insane detail? None of the FRCS releases in the past, not even the boxed sets of yore or the fabled 3e and mangled 4e dealt with those aspects of the world and in 2e they were essentially their own settings with minor crossovers. Maztica hasn’t even had a product focused on it since the original material back in the early 90s! Al Qadim hasn’t been covered since the original limited series material back in the mid 90s. Kara Tur since the earliest 2e days when they did the Kara Tur set. So no, it’s an invalid argument and I think most who make these arguments know that is an invalid argument.
Greyhawk books don’t cover the rest of the planet outside of the Flanaess. Eberron doesn’t go into much detail on the other continents outside of Khorvaire.
The 3e book was a more than adequate overview of the setting in 320 pages. The earlier boxed sets in much less. The 1e and 2e boxed sets spent less than 200 pages describing the setting and that’s not accounting for rules explanations and such included in the DM books of those sets. The 2e set includes a 90 page book covering Shadowdale alone for a total of 350ish pages which is about the equivalent of a post 3e campaign setting book.
Not to insult, my first line was being a bit silly but it is disingenuous to continue these arguments when it’s blatantly obvious when people say they want an FRCS book that they mean something along the lines of the core boxed sets or the 3e book and not some comprehensive encyclopedic 800+ page book. To continue to use it as an argument against such a book really is a bit groan inducing. The SCAG is as much a treatment of the Realms as Waterdeep & The North from back in 1989 was a full treatment of the Realms or a campaign setting. It’s not. It’s a decent region guide, a gazetteer, with material that should be in a full FR treatment to make the Realms kinda, sorta useable in 5e. As a region guide it scratches just enough surface to run the campaigns WOTC publishes and do some very shallow FR campaigns in a very small region.
I get it, for Realms haters there must be a bit of delight in the lack of Realms branded products in comparison to 2 & 3e or even 4e. I was one of them in 2e. Haaated the Realms. It killed Greyhawk!! But to continue these arguments against the idea of one is a poor argument because it’s not what people are asking for. The small gazetteers in the adventures are not what people are asking for. Buying an adventure at $50 a pop for 32-50 pages or so is fiscally irresponsible in a now devastated global economy. A full blown campaign setting sure.
I mean, the most ardent defender of FR here (who I believe has recently left this forum, as I can't summon him), literally does want an encyclopedia of Realms information that is 800+ pages. And I've seen several people on the "What about the Realms???" literally ask for that thing. The poster above me has literally said they would buy 10 boxes on different regions of the Realms.
Now, I'm not a Realms hater, I think it serves it's purpose. I just find the argument for an entirely new Realms Setting book a weak one. In fact, I'll compare the SCAG in page count to Eberron;
SCAG:
Preface and Chapters 1-2 (Setting Material): 95 pages
Chapter 3: Races (Character Options): 15
Ch 4: Classes (Character Options): 20
Ch 5: Backgrounds (Character Options): 9
Eberron:
Preface (Setting Material): 11 pages
Chapter 1: Character Options: 81
Ch 2: Khorvaire (Setting Material): 61
Ch 3: Sharn (literally one city, but still setting material): 29 pages
Ch 4: Building Adventures (Adventure Toolkit): 77
Ch 5: Treasures (Magic Items): 6
Ch 6: Friends and Foes (Statblocks): 37
If we actually compare these two back-to-back, SCAG actually has
more setting material, especially when you consider that 29 pages are devoted entirely to one city, Sharn. The bulk of the Eberron book is coming from the Character Options (way more than in the SCAG), it's adventure toolkit, items and statblocks.
Now I know someone will say, "But FR needs all that stuff too! So it does need a new book!" Except you already have that; there are adventure toolkits and new items and new stablocks for FR all over published material, as every single adventure book and two starter boxes are all set in FR. The material is there, it's just scattered over several different books. Collected, it overwhelms all the Eberron material.
I'll reiterate that I think Laeral Silverhands Explorer's Kit should be refit into an actual boxed set for beginners, adding more character options and an original adventure, for new players to quickly jump into Realms games. But I also believe that future adventures are going to keep releasing FR material, and that it's fans should sit happy with all the content they're getting as D&D 5E default setting.