Simple, Win10 is backwards compatible with DX11 and lower, therefore there are 100% DX11 gamers and only about 25% DX12 gamers. So if there are lets say 500 million gaming PC's, about 125m would be DX12, while all 500m would be DX11.
Further the DX12 is harder to implement, its more time consuming, it's harder, its more tricky. Because its low level developers would need to optimize for each individual hardware, considering there are at least 30 different graphic cards just from the past 3 generations alone, lets not forget intel integrated and amd apu's that can be up to 50 graphics just in the the 3 generations. Optimizing for each one and setting up your engine to work the best with all is impossible.
What we can hope is that there is going to be higher level coding that works for 95% of graphics, and good amount of headroom for devs to optimize for specific graphics, for example for the latest models like the 1060 and higher and 470 and higher.
But again its going to be at least a year before we see fully DX12 capable engines, fully DX12 capable games around those 100% DX12 engines and where the game is optimized for each individual hardware to a good extent. Again this is a big issue, in that it takes a lot more time and resources.
So are we going to see wide spear adoption of DX12 ever? I don't think so. I think its going to coexist with DX11 for the foreseeable future. At least 3 years where DX11 and DX12 will coexist and DX12 won't be the primary API.