You're talking about a race being "unique" as if that is in and of itself a strength rather than a weakness. If they are unique it means that they aren't that popular and no one else wants that space and although this may be justifiable for e.g. plasmids it means that for a humanoid race they are very niche.
mate, did you not see the amount of 'SLIME PEOPLE' madness that folks on the internet went for Plasmids? Folks gonna go
crazy for them once Spelljammer's out
The gnomish subraces having differences between them is partly because forest gnomes are just halflings and partly because there is no thematic consistency and coherence to them. They aren't there because they are wanted. They are there because people are scrabbling around for something, anything, to do with gnomes because they've been in D&D since the beginning when they were off-brand dwarves. And once we've thrown off-brand dwarf out of the gnomish archetype pretty much all we have left is off-brand halfling and one note joke race that would be far better worldbuilding tied to a specific fantasy university (or even to a specific subclass) than it is by making one of the subraces into a risible self-destructive monoculture.
Here's the problem though, gnomes have a bit of an identity, its just WotC won't lean into it harder.
Forest gnomes are your David the Gnomes. They should just be able to talk to all animals because, why not? Really gonna be a danger to talk to a bear?
Rock gnomes are you tinkers, and consumed the tinker gnome in its entirity. Just, lean into that. I mean, they could be with, Spelljammer and Autognomes being a race there
Deep gnomes are the one friendly Underdark race with camo powers
What are the gimmicks for halflings? Describe Tallfellows or Stouts without using "Elves" or "Dwarves", because you could literately just call them "Half-elf halfling" and "Half-dwarf halfling" and nothing would change. When two of the longest lasting subraces of something barely have any identity to them, is the soltuion really to break apart the other thing that at least has some identity? As mentioned earlier, I give Ghostwise their originality (well, i mean, they're just Elfquest elves and frankly should have been elves just because, but, whatever), but when 'Faerun specific random subrace'
You can come up with multiple gnomish villages and have each feel unique. 90% of what I've seen for Halflings is "Its the shire, again" or "Its the halfling mafia, again"
If we look at the existing gnome subraces in 5e we only have three (plus the mark of scribing). In the PHB we have the thematically unrelated discount halflings of the forest gnomes, and the bad joke of the rock gnomes. And then we have deep gnomes who just have better darkvision and better hiding by rocks. Once they stopped making underdark races more powerful there should have been no reason for them to be a separate subrace other than that both the core subraces are so bad.
mate. Its Dungeons and Dragons. The series that, after the good-in-idea, bad-in-execution first race idea, decided that the second most evil race they could make for the Book of Vile Darkness was "Halflings but they're jerks". The series with five different frog people, three of who are evil. New races and stuff to play is what people want and what people actively seek out
The interesting thing about gnomes popularity in 5e is that they are one of the most powerful races in the game thanks to their magic resistance - and despite that they are the consistently least popular PHB race. The most popular gnome subrace (rock gnomes) are based on a bad joke that's bad worldbuilding, and I can't even find Forest Gnomes on the list at all. Deep Gnomes are more popular because they aren't just halflings. So if we removed one core subrace then the game's worldbuilding would be improved - and if we removed the second core subrace no one at all would care. And Deep Gnomes are at least something.
Halflings have lucky. Like, if we're talking 'powerful races', Halflings have Lucky. Also if talking powerful stuff, I'd be putting the original Yuan-Ti and Satyr's proper magic resistance well before gnomes
Also, I doubt the worldbuilding would be removed from removing one longstanding race, and we have a blatent showing of such given how much people complained about gnomes being gone in 4E. Remember? They tried to remove gnomes, and people complained. Roll them into halflings, something that are characterised as nothing more than just tiny humans and have nothing to do with gnomes outside of being short? People will absolutely complain further.
Frankly, I think you'd improve the worldbuilding far more by removing Half Orcs and adding in Goblins and Orcs as core races because then you get your scrunkos and a bruiser race that doesn't have the half-orc's baggage
(Also, as for why D&D's maligned gnomes for so long, my go-to on such is always going to be the sheer avoidance they ever did of ever mentioning fey in the past. Mind, may have been the attitude at the time given nowerdays 'Terrifying fey lord' is something of respect)