Because they're missing an ultra-wide DX lens that has a constant f/2.8 aperture. Sigma's probably still selling about as many 10-20mm lenses as they can make and the 12-24mm isn't that stellar.
A 28mm lens is only slightly wide on FX, and we have yet to see how well those are going to sell. It makes more sense to either update the 50mm and 24mm lenses. It probably also makes more sense to do a 30mm, since that'd compete with the fast Sigma if they decide to get back into the speed game now they've reportedly gotten high-ISO sorted out, but they may decide that's good enough.
I'd rather see an update of the venerable 20-30mm f/2.8, but I suspect that range will lose out to something longer.
I'd rather see them license AF-S to Zeiss at a good price.
Overall though, I'm not sure it makes too much sense for Nikon to update all the old primes- they're not likely to be huge sellers and they've been overwhelmed by production issues on the consumer superzooms- better pro zooms are more likely- the 200-400 was a big hit, despite its slow speed- trying to recreate that success at the wide and medium portions of the line makes more sense IMO.