Because their chart is determined by benchmark score / current sale price. The 1080 wasn't heavily discounted when the 2xxx series was launched, and high end GPUs are rarely value for money winners anyway as cost scales more than linearly with performance. Double the $ might get you 25% more frames.

For context, their page on the 1080 (https://www.videocardbenchmark.net/gpu.php?gpu=GeForce+GTX+1...) gives it a score / $ ratio of 20, while the lowest item on that page has a score of 30.

You could argue they should have some minimum threshold for performance before including them on the chart, certainly no one should be considering a r7 260 or gtx 770 for their new system, however cheap they've gotten, but they don't.

probably has more to do with their price scraping system than it does with the card's performance