People expect their WebAssembly applications to run on more platforms than just Microsoft Windows. There is a reason why web browers have their own graphics API for example. DX12 (and previous version) is the only first class API on Windows. Metal is the only first class API on Mac/iOS. Vulkan/OpenGL are only first class on Linux (and friends) and second class on Mac/Windows. So which API are you going to choose? You'll have to choose the most popular, all or a newly created API that is first class in every compliant web browser. The same thought process happened with I/O devices and WebAssembly itself.