Because it's incredibly risky/difficult. Only a limited number of people can write low-level code like that correctly and not shoot themselves in the foot in some manner. You basically need to be a guru-level programmer to properly handle D3D12.
Which is a huge problem, given that most developers want to spend the absolute minimum on the PC version. Plus, it requires Windows 10 and there's no way to forsee how well that will be adopted. OTOH, it could go the other way if it's easier to do DX12 only titles by porting the XBox One version.
So it's not about more effective API - DX12/Mantle/Vulkan/Metal just give developer tools for more effective (low level) but hard programming? Than opposite question: why should developer use these
incredibly risky/difficult tools for PC and mobile games? We know their prices are lower, so it doesn't seem reasonable from commercial point of view.
The only exception might be DX 12. If Windows 10 is a single OS for all platforms (PC, console, mobile), then there is no need for porting, is it? It also should be easier porting on Android, as Vulkan is going to be single API for desktop and mobile platforms, right?
Or perhaps it's just impossible to develop optimal game for specific OS - it can be done only for specific model of PC/gadget (when developer knows which CPU and GPU is used in it)? But it seems to be wrong guess - 3DMark API Overhead + DX 12 is effective on any top dGPU and iGPU.