Because Apple. That's a pretty damn compelling reason.
To be clear, I don't think it's smart to completely discard the existing work that's gone into the modern standards. The point is that we can't do Vulkan in its entirety without explicitly excluding a large chunk of the web. We could do Vulkan but with several functions stripped out (or made into extensions), a different model for memory management, and some limits on shader capabilities. That's not really Vulkan anymore, though, and if we've already made the call to break broad compatibility with the existing API then it's probably worthwhile to see what we can do to make it feel more at home on the web anyway.