The primary benefit was so the people who did the Complete Book of... series for 2e didn't have to come up with completely different histories, mythologies, kits, etc., all on their own.
Neither race has a lot of mythology that could be easily plundered. For halflings, you get hobbits and, if you really feel like it, Munchkins from Oz. And that's it. For gnomes, you get Dragonlance tinker gnomes, that book by Wil Huygen and Rien Poortvliet, gnomes as creatures of elemental earth, and gnomes as being the same as (mythical) kobolds--but the latter two would go against how D&D had always treated gnomes. Dwarves and elves have tons of stuff in mythology, fiction, and in previous D&D lore. (I remember first reading the Complete Book of Elves, seeing the two-year pregnancies, and thinking, somebody's been reading ElfQuest. 2e had a lot of cool stuff, but they weren't really as innovative with the PC races as they could be.
Honestly, I think it could be cool if a hypothetical 6e had gnomes as being creatures of elemental earth. They don't need to be any more elemental than the genasi are, but they could be something like halflings or dwarfs who emegrated to the Plane of Earth eons ago and adapted.