After all, the PCB for GeForce 8600 on down is based on the stable GeForce 7600-series line, which was already a derivative of the GeForce 6600-series layout. The only component that would need any fine tuning at this point is the GPU -- and that's been stable for months.
One possible reason is the recent delay
of AMDs R600 generation to the second quarter of the year. There is no pressure from the competition to rush out the
cards to the market. Instead NVIDIA can lean back and happily sell
current GeForce 7 mainstream cards. Meanwhile their GeForce 8
mainstream cards based on G84 and G86 are in production. Basically
NVIDIA is preparing for another hard launch.
According to industry sources I've talked to, NVIDIA will be able to deliver enourmous amounts of those
cards when they launch - they will basically flood the whole market.
If you talk to the AMD crowd, however,
you get other reasons, which are also somewhat realistic. Apart from
various problems with the hardware, NVIDIA would delay the launch as
long as possible to have a working driver ready. The
current state of G80 drivers is not at the level of quality
one would expect.