"We don’t expect this to become the actual API that ends up in the standard, and maybe not even the one that the Community Group decides to start with, but we think there is a lot of value in working code. Other browser engines have made their own similar prototypes. It will be exciting to collaborate with the community and come up with a great new technology for graphics."
Also, Metal was announced June 2014, released September 2014. Vulkan was announced March 2015, released Feb 2016.
I don't see any reason for favoring Vulkan specifically for Web when compared to Metal I can read: "Metal still does a lot of hand holding and behind-the-scenes management for you, while with Vulkan you are responsible for — literally — everything. And man they were NOT kidding when they said that the API is explicit. Its actually quite ridiculous how difficult and detailed the API is." (1)
Also, Metal was announced June 2014, released September 2014. Vulkan was announced March 2015, released Feb 2016.
I don't see any reason for favoring Vulkan specifically for Web when compared to Metal I can read: "Metal still does a lot of hand holding and behind-the-scenes management for you, while with Vulkan you are responsible for — literally — everything. And man they were NOT kidding when they said that the API is explicit. Its actually quite ridiculous how difficult and detailed the API is." (1)
1) https://forums.developer.apple.com/thread/38469