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

Yep. Sales statistics for ILCs are rather revealing in this regard. The average DSLR purchaser - of which 80% purchase DX equipment - owns only 1.5 lenses. This means, practically speaking, that most DX purchasers own only the kit lens that came with the camera and at most 1 more, most likely a slow telephoto or cheap standard prime. The 18-55 and 55-200 pair is well represented.

I'm not sure that Nikon was misguided in selling the 17-55 f2.8. When it began design, around 2000, the D1 wasn't out yet and FX was still a distant gleam in the silicon foundry engineers' eyes. It was an excellent "hero" lens at its 2003 introduction and did a good job attracting the digital early adopters considering the D1 and D100. Nikon probably planned to produce more gold ring DX lenses, but sensors got more affordable more quickly than they expected (the Canon 5D came out in 2005), and the real money was to be made in catering to the frustrated point-and-shoot upgrader market that Nikon so masterfully mined with the D40. In today's market it's somewhat of a beautifully performing throwback and sorely challenged by Sigma's and Tamron's offerings.

Cameras like the XZ-1/2 and LX7 show that purchasers do appreciate fast lenses for their low light capabilities, but only if that speed is convenient, and that's only possible with a smaller sensor. Fast lenses and small sensors also work because they're effectively working at DOFs equivalent or larger than the slow consumer DX zooms and don't require the additional effort at focusing and composition that the fast lenses do.

At this point someone who really wants to shoot with f2.8 lenses and retain the compactness advantages of DX would be better advised to switch to a u4/3 system and avail themselves of the very compact Panasonic 7-14/12-35/35-100 trinity, the last two of which are f2.8 and the first f4. The sensors being used in the latest Oly cameras are very close to the image quality of a DX D90.