I see three big factors for lower sales

1) Economy. Really, people would overlook the other 2 when they didn't mind spending a few extra bucks, but well... now they do.

2) "Casual games". I don't know if there's more actual casual games than there used to be (I think there are) but there's certainly more awareness of them... there's still going to be games that push the graphics to the limits, but there seem to be a lot more games that basically have "good enough" graphics and focus on the fun and gameplay instead. This will lower sales, gamers may think twice about whether it's worth spending cash on a new video card just to play a few extra games.

3) Moore's law. Any more, even some crap on-board video chipset is good enough to play some reasonable fraction of the graphically intensive games and plenty to do everything else. The days of the "Intel Non-Extreme graphics" (I810 for instance, which barely seemed to handle a 3D screensaver) are past*.

*In most cases. I guess some file and compute servers and the like still come with oddball video chips, figuring the machine will either be headless most of the time or have a Quadro dropped in for serious 3D work.