If elves were given one niche over the other (magic vs nature), there would have been a lot of complaints about WotC "ruining my view of elves!" but there is enough distinction between the two types to make them very different (they don't want the moon/sun elf difference - races are meant to have more character impact than just some different stats).
They made two races to satisfy people, and to me, that makes sense.
Gnomes have had a few niches in D&D. Sometimes they are close to nature (speak with animals - the lawn gnome type like in that old Nickeloden cartoon), other times they are the science guys (either alchmical or engineering as in Dragonlance), sometimes they are a very magical race (great with illusions), and they most recently they are the pranksters, jokesters, and singers. Even in 3.5, they were muddled: illusionists (+1 DC), bards (favored), achlemists (+2 craft), speak with animals (1/day), meele (bonus vs golbins and kobolds, and trained to dodge giants), and dwarfy (+2 con). They are a mix of every gnome stereotype, and can't focus on anything.
Gnomes could be split into several racial niches:
Natural - but elves got that
Magical - elves got general magic, but gnomes could get illusions/shadowy, but the PH1 doesn't seem to be focuses in on that, so why have a race good at something that there is no class for yet?
Science - again, Science as a power source is not core. WotC might do a book about it later (Eberron, after all hinges on a lot of science), but they didn't want this to be PH1 info. If they don't do it, surely a 3rd party pub will, and gnomes might take center stage in such a book.
The bard/trickster - people seemed to hate that when it came out in 3.5, but maye it would work. Oh shoot, bards might not be PH1 either! Once more, a class that would play to this gnomish niche might not be PH1 so why have the gnome there?
Could WotC have banged out a gnomish niche? Yes, they probably could have done so for release, but they would have needed to bang out more than just the racial niche, but the rest of the class niche that the gnomes would fill. Why have a good illusionist wizard if illusions are not fully fleshed out? Why have a great tinkerer if science is not fleshed out? Fleshing out 1 race might have been possible, but that race would have sat in a void.
I say let the gnome come out when there is enough crunch and fluff to support the whole race (possible two races: illusionist and tech).