Because the uneducated will still pay more for old products. Just like with the 8800GTS 320/640MB. People see 640MB and think, "Gee, this has to be better than this 512MB GTS. It's only $30.00 more, what a deal!" *shrug* I've seen it happen a few times already.
Also, with the price gap being so little between the 3870 and 8800GT (512MB) it's hard for me to recommend the 3870 any more. Although, the 3870x2 is still a great item...