That's because the lion's share of NV cards on Steam's survey are previous generation, which are lumped in with "on board graphics" in this poll. That's where you find all of us with 8800GT through GT250s. NV sold those in rat horde numbers for about two years while ATI had no competitive part. This forum was wearing "green" glasses during that time.
For 2009 the value offered by the 4 series was a no brainer. However, the 8800GT continues to be "good enough" for all but very, very few titles and many of us are sticking with one until pricing on the 5 series stabilizes and/or NV comes up with a compelling upgrade part.