It mainly comes down to cost. To place a portrait grip on the camera, you are essentially putting in double the components, two shutter buttons, two terminals, all the wires for the extra functions of the other buttons on the grips. You also have to consider how much more gets crammed into a “High Pro” camera like the D3. It has more CPU’s, thus needs more space for cooling and for the CPU’s themselves.
As you add all this stuff, the price just keeps jumping up and up, the resulting end a camera that cost as much as a car.
If you were looking to go pro, which would you rather buy?
a D300 and a vertical grip plus another lens for 3,000$
or just the D3 camera’s body for 5,000$? No lens included.
It also has a little to do with having pro lens too, for example, I use the 200mm f/2 and have used the 600mm f/4 Nikkor lens. Needless to say one weights 10 lbs. the other weights 17 lbs. If you tried to shoot a vertical portrait with that much weight on your hands or even your mono-pod and didn’t have the grip, your arms would explode.