Nvidia's GF100 Turns Into GeForce GTX 480 and 470
And you know who buys the top of the line super expensive cards? Pretty much no one.
Then why can't supply satisfy demand? Prices on all the enthusiast-oriented cards have been going up for months, and if you really want a top-end card (5970 for example), it's really hard to find one.
Monitors are getting bigger and cheaper, and a lot of people want to play at (minimally) 1900x1200 at high settings. For newer games that takes expensive cards.