It takes time to implement it in any API. With DX9 and DX11, Nvidia assisted game developers directly. With DX12 mGPU, Nvidia straight up said that game developers are on their own - and that's why we have so very, very, very few DX12/Vulkan games that support mGPU.
Time = money. Game devs (for the most part) don't care enough about the 1% or less of the gamer population to warrant the cost to them.