Two main reasons spring to mind. First, the smaller view in the viewfinder when shooting DX. Second, I have no interest in processing the huge files in FX.
Also, the large number of purchasers who reported problems with the focusing coupled with the fact that the D800 body is a diffferent (smaller) size and has a different arrangement of controls and totally different batteries and chargers. I currently use grips on my D300 and D700 that use the Nikon EnEL4A batteries as do my D2h, D2x and D3s. I find that interchangability to be of immense importance.
I am sure that Nikon's intent was as you describe which is why we have yet to see a D400.
Naturally other's usage of cameras will differ from mine and I do think that Nikon's accomplishment with the D800 is astounding.