WotC has made it quite clear that they have no intent of publishing a complete setting book for settings that are well detailed in one or more prior editions. The two big objections to that are 1) new players don't even know these settings exist or that they could look in previous editions and 2) crunch needs conversion.
I don't have SCAG, so my feelings are based only on what I have read about it in these forums, which isn't little however.
I am not so sure what is meant by "new players". Real beginners start in someone else's gaming group and don't buy anything for a while until typically they buy the PHB, or they are people who first buy the PHB (or a starting box) and then join an existing group or start their own. I don't expect these people to start buying supplements for a while, at least not for the crunch (what is the point in wanting new subclasses and backgrounds when you haven't even played more than a couple of the core ones?).
OTOH,
if they are DMs then they might want to check out some fantasy setting fairly soon, in order to be able to 'stage' their adventures somewhere. The PHB+MM+DMG don't provide setting information, only a certain 'vanilla' implied setting with all the monsters existing for example. So for those people I could imagine they would like to buy a book like this (where the extra crunch could be useful to give setting cultural features a more 'substantial' representation, that the DM can use for NPCs or propose to the players), and being maybe only a few months into the game, it makes sense that a
cheap (i.e. slim) product is financially more attractive than a huge tome.
As for me, when I was a "new player" myself (2e era), what fascinated me most were exactly the huge tomes, not slim ready-to-use digests. I saw a lot more value in an encyclopaedia that I could read for years (I still do...) and use to generate endless adventures, and bring a whole new dimension to our tales. But maybe that's just me.
Note: I am not saying I want WotC to publish a new 5e FRCS. I already have a FRCS + a few additions, as well as other settings books. Probably I don't need more. And I might even buy the SCAG anyway since it's just so cheap, but it'll be for the little crunch, and just because.