Why hasn't Nikon released a f/2.8 DX zoom?

I have been thinking about that as well, and feel like the writing is on the wall.

The DX crop factor does not get you much in terms of glass/materials/manufacuring cost savings. Most is probably in the normal range hence the 17-55, and even then compared to the 24-70 the delta is negligible. A DX wide angle is difficult to correct, and a DX telephoto would be just as big and expensive as the FX. Especially if you want a PRO level lens. The cost savings would be negligible and DX users are generally plenty happy with the FX lenses.

Look at the lack of a D400, the lack of any worthwhile DX lens announcements, the D600. Nikon was late to the game with mirrorless so the DX SLR market is probably stop gap measure untill they move APS-C into the mirrorless segment and all SLRs become FX. the pixel density of the new FX models give you plenty of cropping capability. Sure there are some other factors like fps and what not, but reallity is that DX PRO is probably dying. If you spent a ton on those lenses you would be pretty ticked.

And besides, Tamron and Sigma have happily stepped into the DX 2.8 zoom space, even in the 600 range you can get their comparables of the 17-55