because the g92 chip has a 256mb memory interface. You have to go in mulitples of that. Also, its not so much the amount of memory on the GTX that helped it, but the memory interface. By dropping down to the 256mb interface the chip costs a lot less to make, and yields come up. They are getting more performance out of a smaller interface by spec'ing the memory higher. So lower costs = lower prices to the consumer. They sell more cards at the same profit margin thus increasing profits.