[mattnewport]

There are several good reasons. D3D12 was finalized some time before Vulkan and was available in early / preview versions for quite a while before that. Stable drivers were available much sooner than for Vulkan. It was an easier transition for code bases that already had D3D11 support and Microsoft offered additional code to help with the transition (the D3D11 on 12 stuff). You could bring your existing HLSL shaders over easily. There were better debugging and profiling tools available sooner. The Xbox One got some additional extensions to better utilize its hardware.