Because Windows 8.1 has an extremely small user base (and is still a massive cluster), especially among people who buy graphics cards to game with. It's the same reason nVidia killed Windows 8.1 support with the RTX series. But really the Fury series hasn't had performance improvements in some time anyway, sadly, so you're not missing out on anything.
That was Steam's Hardware Survey, gives a decent cross section of "gamers", but let's use statcounter, which is more favorable to you, Windows 8.1 has 6.67% of the market worldwide, that's a very small amount of people, especially when you consider all the laptop and OEM desktops included in that figure. Windows 7 driver support will be terminated in January 2020 when its drop dead date is reached on the 14th, so there will only be Windows 10 and Linux drivers then.
