One of the problems there is that you can't just spend all of your (characters') time in space...it'd be like having sea-faring characters who never sailed into port. Spelljamming characters have to make land-fall on a planet sometime, and then it becomes rather hard not to mention that the other game worlds' planets are out there.
Another aspect of SJ that seems to be largely ignored is that it often got seen as an extension of a planet-side campaign. Realms characters could get a spelljammer and never want to leave their sphere, or even go too far from their planet. For them, spelljamming was just Forgotten Realms in the sky, etc.