I doubt those vendors really want to retrace their steps to go through the multi-year pain and many committee discussions that brought us SPIR-V, again. Look at all those names on the SPIR-V specification. Do you want to waste all those people's time, again, for that many years?
Nobody inside the WG but Apple wants WSL.
On the other hand, the SPIR-V working group has no participation from Apple or Microsoft (vendors of relevant platform APIs, relevant shading languages, and in Apple's case, of relevant GPUs).
You might argue that Apple should join the SPIR-V group but you could likewise argue that AMD and Nvidia should join the WebGPU group. After all, their voices would be valuable as to the API and not just the shader language. The barrier to entry to a W3C Community Group is also much lower.