Probably the same reason it has no flash. It assumes you'll add focus assist with an SB-600, SB-800 , SU-800, or SC-29 if you're in a low enough light situation where you would need focus assist lighting.
Guys that is SHOCKING, the main reason I am interested in the D3 is because of its low light capability, I am not a big flash fan, so does that mean the D700 is more capable under low light?!!! The D3 is bulky enough, carrying a flash to focus is just plain silly, did anyone compare the D3 under low light with a D300 for example? I find the beam light VERY helpful in those situations with a 1.4 lens, the d300 can focus in pure darkness with the light beam
I definitely see your concerns. Hopefully it can focus in dark enough environments that if it can take a decent photograph it can focus. You might have to find something a bit more contrasty to get a good lock, if you're seeing issues.
Maybe somebody with a D3 can ring in. I do know I haven't seen anybody ever complain about that as something missing... and a lot of people have a D3 for low light shooting.
The D3 has unbelievable low light focus ability in dark rooms without the AF light. If there is any contrast at all it can grab it. During my last two dance recitals with many numbers in near dark lighting I only recall AF hunting 2-3 times during two days and nearly 9000 shots and probably 12 hours of shooting. Lenses used 24-70 f2.8, 105VR, and 200VR all shot wide open, ISO 3200 or higher, and 51pt 3D AF,
The d3 locks on in low light pretty well without it. Almost as good as it's canon counter part the 1Diii. I haven't used any other nikon cameras but both the D3 and 1Diii lock on better and quicker than the 40D or 5D
I have not had any issues shooting from the sidle lines during 7 on 7 football in some bad lighting here on the northside of Houston. I shoot at M 800 f5.5 or greater and auto ISO. No issues from the D3, The camera just dose what you ask it to do!
I agree with some of the other replies you got: the D3 does a remarkably good job of focusing in low light (and I hear it's even better with firmware update 2.0). When I've had problems with low-light focusing it's been when the light levels are ridiculously low and I've had to shoot at the highest possible ISO settings. That's really pushing it! The other times when I've noticed difficulty in focusing is when I've used a lens that is notoriously inclined to "hunt," anyway. For example: the 105 VR macro, with its focus-limiter switch not engaged -- when used for taking head-shots in low light. That's just asking for auto-focus problems. It would be interesting to know how much better auto-focus in those situations would be with, say, the 85/1.8 (which I don't have yet but which continues to tempt me).
Overall I've been very impressed by its ability to focus accurately in low light -- the D3 is head-over-heels better at this than the camera I was using before (Canon 5D, which is certainly a good system in some other respects).