By the same token many card won't fully support DX11 then either. Devs will take the least common denominator (ie consoles). That's a good place to start.
Extra features are only useful if people use them.
Well, the Xbox One and PS4 are DirectX 12 capable consoles, but the problem is that most of the benefits about DirectX 12 come from CPU limited situations, such as consoles and current horrid (in comparison to Intel) AMD processors (which should change with Zen). You take a current i7 and a Fury X or 980ti, and it's not going to matter if it's running on DirectX 11 or 12, it's not going to make a difference. You take an APU in a console, then yea, it's going to matter, and that's what Microsoft is hoping for because it went cheap with its RAM and it can't handle 1920x1080 content.
And then there's two little words which break whole reason for high frame rates: Adaptive Sync. Be it AMDs FreeSync or nVidia's $200 G-Sync, they're much more important than the 144hz panel marketing hype. And let us remember, there are still 0 PC games which use DirectX 12.