It would be good to remember that these products are doing different things.
Radiant Citadel introduces us to an entire setting and presents a series of adventures n that setting, and the highlight of that setting is how different everything is in each segment of it. They're covering massive ground, and it is mostly entirely new ground.
Spelljammer, however, is updating an existing and well supported concept. Do I need a battlemap of a Nautiloid in the product? No. because I can use Google and find dozens out there. Do I need lengthy ecologies for Neogi, Giff and Thri-kreen? Only if they want to make them differ from the lore established in the 1990s. Otherwise they give us the minimal version and we Goggle the rest. Unless they're changing the lore, the majority of the Spelljammer lore can be presented briefly in the books to facilitate Google searches (or just flat out DM's Guild purchases of old material - I could 20+ products there).