Like Caliban, selling Spelljammer is really about selling the DM's vision of Spelljammer. There's a lot of stuff mixed up in Spelljammer that isn't for everyone. The silliness in particular isn't to everyone's taste. And frankly it's hard to run a campaign with both believable Neoghi and believable Tinker Gnomes and Giant Space Hamsters (as written).
My sales pitch (because it would be how I run it) is it's D&D's version of Age of Sail Colonial Empires In Space. The Elven Space-Empire plays the role of the British Empire at its height, with areas it controls thoroughly, colonies it nominally controls, and plenty of areas it doesn't control at all (but can exert gunboat diplomacy). Most of the campaign takes place in the second two areas.
There's also the Dwarf Colonies on Volcano Worlds, Mos Eisley, Babylon 5, Alexander Quartermain, White Apes, Space Reavers, Giff Mercenary Armies, and a Princess of Mars.