Its the same reason I doubt you'll be seeing any companies opening their hardware anytime soon, as AMD bent over backward, even hiring coders to help the FOSS driver guys and opened their specs as wide as they could, and what did they get? every forum filled with guys saying "Herp derp, buy Nvidia"
With regard to GPUs, I currently have a (aged) Nvidia GPU but my next GPU will be the top end Intel Ivy Bridge. I'll be going Intel because I want a newer and faster CPU, the Ivy Bridge GPU will be fast enough for me, and most of all because the open source Sandy Bridge and Ivy Bridge support from Intel is strong now and improving. Intel seem like they'll hit the ground running for Linux support when Ivy Bridge is released. I want strong, out-of-the-box, open source GPU drivers for Linux and that's what Intel will deliver.
Here's a recent article from Phoronix which bencmarks Intel's progress with its Sandy Bridge\Ivy Bridge drivers for Linux: http://www.phoronix.com/scan.php?page=article&item=intel_sna_maturing