DirectX 12 isn't any where near ready for launch. Their current time table seems to be an early 2015 release for the DX11 client, and DX12 won't be ready until Holiday 2015. You'd rather have them wait up to an extra year?

Also, you need windows 8 to use anything past directx 11.1. Meaning 12 will definitely require windows 8+. Directx 11 can hit a larger number of their fanbase.