Because, from where I'm standing, gnomes have zero presense. No major elements in most settings (Eberron and Krynn being exceptions)...
This isn't true.
In the Mystara setting, the gnomes built the flying city of Serraine, the aircraft used to defend it, and if I remember correctly make up at least the bulk of their pilots (and possibly all of them).
In the Hollow World, a sub-setting of Mystara, two different groups of gnomes were able to build flying islands that crashed into each other to form a single flying island, Oostdok, with each group blaming the other for the disaster. The gnomes of Oostdok also have a fleet of dirigibles.
In the Forgotten Realms, in addition to the normal groups of gnomes that are scattered across the world, there is a large concentration of them on the island nation of Lantan. Although Lantan has never gotten its own supplement, it's not infrequently mentioned as the source of wondrous inventions in other Forgotten Realms materials. (For example, the submarine in
Waterdeep: Dragon Heist was crafted in Lantana and it has a crew of Lantanese gnomes.)
I'm not extremely familiar with
Spelljammer, but I do have the initial boxed set. I remember that gnomes were one of the relatively few races to have at least one unique craft of their own, and I'd be surprised if they weren't well represented within the setting.
Gnomes are relegated to the background in Greyhawk, but that's more or less true of all the demi-human races in that setting, since Gary Gygax made no secret over the years of wanting human beings to be central in the setting. Even so, svirfneblin feature significantly in the D portion of the epic GDQ series as potential allies.
None of these represent
essential components of their settings, but they're not exactly insignificant either.