WebGPU is for the Web. You're expected to be able to put up a webpage and have it run everywhere (as much as is possible). Vulkan is designed so you have to make paths for every differen type of hardware. That's a non-starter for the web
The intersection of hardware features is greater than the intersection of Vulkan, dx12 and metal features.
Moreover, hardware features should be queryable at runtime
That's why it's versioned..
Webgpu could start from Vulkan 1.1 or the latest Vulkan version could be bumped to Vulkan 1.2 for marking it's the one used for webgpu.