I can't grasp the need to do anything with gnomes or halflings. If someone wants to play one, they can play one, otherwise they can be simply ignored.
Space efficiency. As I mentioned upthread, my goal* was to come up with exactly 4 variants within each option. I hit a dead end with gnomes, but after wracking my brain about them for ages, I realized that svirfneblin and ghostwise halflings are surprisingly similar, and forest gnomes and lightfoot halflings had some similarities (even moreso if you consider lotusden halflings). If one ancestry had those four variant lineages/clans (forest gnome/lightfoot halfling, stoutheart halfling, rock gnome, svirfneblin/ghostwise halfling), it would slot perfectly into that overall system, preserving most of the unique character of the individual options while streamlining a significant amount of space.
And it's not like I'm
alone in thinking there's a connection here. In Dragonlance, the local equivalent of halflings (kender) came into being, allegedly, from those gnomes who chased after the Greygem because they were curious. (Allegedly that's also how
dwarves came to be, coming from the gnomes who sought the gem because they coveted it, but the dwarves apparently vehemently deny any shared origin with kender, so who knows?) And Dragonlance is by far one of the most influential sources in terms of defining what gnomes are, given that it is the origin of the whole "eccentric genius whose technology is reliably unreliable" trope associated with them.
Keep in mind, I see this as exactly the same as having (wood) elves, eladrin, drow, and shadar-kai all being "one ancestry," despite their significant differences in physiology and implied culture, or the differences between the various types of genasi, or the three extant types of dragonborn (metallic, chromatic, gem). That is, perhaps rock gnomes and svirfneblin/ghostwise halflings are known for being more "serious" (but also a bit more unhinged), while forest gnomes/lightfoot halflings(/lotusden halflings) and stoutheart halflings are known for being more grounded but also a bit more trickster-y. By analogy, very similar to the split between "wood" elves and "high" elves (or just "elves" and "eladrin," as far as I'm concerned); as Red from OSP puts it, "Elves are generally nature-loving hippie archers, except their cousins, the naughty word Elves who are always pretentious with massive superiority complexes." That both things are "elf," despite cashing out in different ways, doesn't weaken either, but shows how both depictions fit into a broader concept or understanding.
*I succeeded on this for every race except human, given the "gnome/halfling" combo, because I couldn't come up with a clear, distinct, interesting,
and most importantly NOT RACIST fourth option for humans. After giving it a fair shake and still coming up empty, I settled for humans having only three "lineages" rather than four, because I'd much rather have them be a little weird mechanically than perpetuate racist stereotypes. That left me with standard, Dual-Blooded (e.g. half-elf, half-orc, half-dwarf, etc.), and SPACE(!) (e.g. slan, Solarians, potentially Vulcans, etc.)