It isn't actually bad, it's just utterly inadequate (and it's very large, elaborate, and expensive while still being inadequate, which didn't help its reception). What there is, is mostly quite ok. I have my issues with some of the player-facing material (I still hate elves having floating proficiencies, and the less said about the various hadozee disasters the better) but it seems acceptable, on the whole.
I loved Elves having floating proficienies. It really sells them as being reincarnated beings. I've been advocating for that to happen more.
As for the expense... I don't see it. It cost about $50, the same as just about every other DnD book. Now, you might argue it was three books, but I think this is a misconception. It was one book, packaged as three. With a high quality map and a DM screen.
You can reasonably argue for not having locations in a Spelljammer setting book, sure. Space is big, and you can't cover it all - thought there's not many excuses for not at least covering SOME of it. But in that case, you need to cover organisations. Tell us who the Imperial Elven Navy (or the neogi, or the giff, or the beholders or even the scro or the Shou trading fleets) are, what they want. How are they structured, what are their goals, their enemies, their leaders, their tactics and strategies, their strongholds, their internal factions, the members of their organisation that PCs are likely to encounter in diplomatic/military/exploratory/assassination scenarios? How does a PC join up, if they're so inclined? What does an adventure involving them look like? Give us plot hooks for these organisations, give us a variety of stat blocks for members of each organisation useable at different CRs. In short, go look in the organisations section of Rising From The Last War, and do exactly what they did there, because that was really solid, useful, practical material for running a campaign.
Um... can you be more specific about which section you are talking about for Rising? Because as far as I can tell, it doesn't do this... at all. I mean, I love Rising, but what you are describing isn't even in there.
I just flipped to the page on Breland. I see the name of their Capitol, some vague details like the Dark Lanterns existing and who rules the nation, some quick and dirty details major locations, and some "if you are from this country" stuff. But how is the country structured? Other than being a monarchy I have no idea. What are their tactics and strategies? Um... spying like everyone else? Strongholds... basically none? Internal factions? No idea. Statblocks? Zero specific to Breland.
I think the big thing you are missing here, and maybe what is going on with your comparison, is the focus of the setting. Eberron is highly focused on the political and social structures of Khorvaire, they just finished a massive war and many many plot hooks revolve around the fact that everything is a powder keg about to kick off the Second Great War. So, Breland's relationship with its neighbors is important, as is how the previous war affected it. Those are the focus of the setting.
But the Imperial Astral Elven Navy (which one?) isn't a focus of Spelljammer. And that "which one" isn't a throw away question. The Xaryxis side bar makes it very clear what is going on. There is not one single Astral Elven Empire that rules all Astral Elves. They are but one. And this applies to the Beholders as well. Why would I need a paragraph telling me ANYTHING about the Beholder navy? They are xenophobic paranoids who can usually not stand the sight of other Beholders. They are highly individualized, every beholder ship should fight somewhat differently, they don't work together in the way you are implying. And so, all you are asking for are examples of specific fleets, when that isn't the focus of the setting.
I would also note something very key with this section of your post. You want information on the Scro or the Shou... but those words are meaningless to me. They don't exist in these books. That tells me you are coming at this with expectations of the previous edition. You are looking for things that don't exist in 5e Spelljammer, and therefore are disappointed because you aren't seeing them.
I'm not even mad about them not including anything about religion, just disappointed. It's annoying, but after they wimped out on covering it in Ravenloft, they were never going to do it here.
What religions should they cover? Why would Spelljammer have different religions than the base gods we generally discuss? I mean, there are tons of deities referenced in the PHB, don't all of those apply and more?
There's lots of classic spelljammer monsters that the weren't included in Boo's book, so that a bunch of last-minute Dark sun rebrands could be shoved in there. Spelljammer and Dark Sun are thematically worlds apart. Belgoi etc fit in a Spelljammer monster manual like a live weasel fits in a caesar salad. If, as you argue, there's already enough monsters in other sourcebooks that you don't need any more here, wind the monster book into the adventure book, don't just pad it out with critters that belong in an utterly different setting (if an anaemic 64 page pamphlet can be called 'padded out')
Again, you are going in with expectations, and therefore disappointed. But someone picking this book up with no expectations isn't going to be disappointed by classic monsters not appearing, because those monsters do not exist to that person. "I wanted to see X" is very different from "this is a bad setting, because it needs X"
Also, no idea what a Belgoi is, can't find that in my book. Nothing in the book seemed like it radically didn't fit. The B'rohg were weird, but their mention of often being taken as gladiators makes sense. In fact... I'm honestly baffled. All of these fit to my eye. Nothing here seems like it doesn't.
The Spelljammer box is fundamentally not what was advertised. It isn't a campaign setting, even though it claims to be. It's an adventure that is dressed up as a campaign setting, like Strixhaven was, and like WotCs preferred model seems to be these days. Radiant Citadel was the same sort of product but to its credit, was more honest about it. The setting material only really exists in a bare minimum form to support the campaign. The great majority of locations it covers are only detailed in the adventure and lack info that you'd need to make them useful outside the narrow scope of that plot, many of the monsters are not Spelljammer monsters and are only here because of the not-Athas world in the adventure, etc etc. Which is ... fine .. if that's your thing. You can play a non-Xaryxis campaign using this book in the same way you can run a non-Strahd Ravenloft campaign using only Curse of Strahd as your campaign setting. Maybe the hags are your campaign-ending bad guys instead of the snappy-dressing guy in the castle. Of course it's possible, but it's not intended, and the product doesn't make it easy.
I'd argue you are utterly wrong.
This setting material is sufficient for me to run a non-Xaryxis campaign from levels 3 to 15 with ease. Now, part of that is going to be creating a lot of NPCs and other content... but that is going to happen with anything.
If I wanted to run an Eberron adventure, set in New Cyre to explore the Mournlands... I'm going to have to make up a lot of NPCs. The only ones provided by the book are King Boranel ir'Wynarn (no details other than he is popular), his children (shrug, they don't even get names), Prince Oargev of Cyre, and the Lord of Blades. I could involve House Cannith I guess which gives me the three barons, again just names.
But those are all LEADERS. So... who runs the inn in New Cyre? Who is the local blacksmith? Who are rival factions exploring the Mournlands? I'd have to make up the majority of the characters in the story, because unless the PCs only ever talk to kings, I have no other NPCs.
So, when I make up who the middle manager of the Docks at the Rock is... how is that any different than making up an innkeeper in New Cyre? When I make up a pirate queen to assault my players as they explore an abandoned asteroid colony, how is that different than making up a Bandit Leader who assaults them as they explore a town in the Mournlands?
Spelljammer isn't a setting about political intrigue, it is a space setting, about exploring space. Just like Star Trek, each place they go can be made whole cloth, and that's fine. It actually gives me some freedom, because I'm not constrained by a highly detailed lay out of the entire universe. What's out there? Stuff like this, which is plenty to get started.
Aside from any/all of that, the format really bugged me. We've been told continually that printing costs are high and printstock is in short supply - so why was this product sold in a format that involved so much wildly unnecessary cardboard? There is FAR more cardboard than paper in the slipcase version. I don't need or want a slipcase, i don't need or want a DM screen. Don't give me all these expensive extra gubbins and fripperies and then spit out a skimpy content-light paraphrase of a campaign setting. Give me useful, practical gaming content, then let Beadle and Grimm worry about all the bells and whistles later. I kinda hate to pan this product as much as I am, because I like Spelljammer and I do want WotC to give us a decent 5th ed adaptation of legacy settings Dark Sun (or even a modernised Al-Qadim!). And the art department knocked it out of the park, the books are gorgeous. But all in all, the whole product is just an overwhelming victory of style over substance, and a massive lost opportunity.
None of this applies one iota to the setting material. This is all just complaints on the product design.