I'm just having a crap load of problems with Win8, I could not set up a network that worked with my Wii U, and more recently I found out that GFWL doesn't work on Win8.
Of course it's not, but it's a mixture of market targeting and complacency. Using DirectX 9 means they can target gamers who are still on Windows XP as well. The devs who have enough time, money and man power will include other render paths as in the case of Metro 2033, which had DX9 and DX10 renderers.
The other part is complacency. Its not that the devs are lazy, its just that they may be more familiar with DX9 and want the option to port to the current crop of consoles. That's why games like Borderlands 2 still use DX9, as do others like Team Fortress 2 (moving to DX9), Minecraft and Diablo III, which runs on so many machines. As we move away from supporting XP to a stronger Windows 7 and 8 install base, these things will change and more devs will jump into DX11. The next generation of consoles are going to make sure that push happens. I don't know anyone who's still on XP, most have moved to Windows 7, thankfully.