I didn't say the SCAG was poorly recieved?
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.
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.