Apple decided to not support Vulkan as it had been developing a competing API called Metal at around the same time. I don't know why they don't also support Vulkan.
But Microsoft isn't supporting Vulkan in Windows either, so why is there only a fuss about Apple's support for it? Both companies have competing API's (Direct X and Metal), but Apple is the only one getting flack for it. Why is that?
For Windows I install a graphics driver by the GPU maker and get Vulkan support (according to wikipedia that is the case for Nvidia, AMD and Intel), since Windows allows drivers to offer such APIs without any specific support by Microsoft. With a Mac all the drivers come via Apple, without support for it.
1) Windows allows the GPU manufacturers to ship their own Vulkan support; AMD, Nvidia and Intel all do this, which covers 99.99999999999999% of all Windows machines where this is an issue. It's not as good as native support, but it's good enough.
2) Windows has over 90% market share on desktop. macOS has far, far less. It's a lot easier to get people to support your API when you have that kind of market share. iOS doesn't have enough market share for devs to ignore Android.
If they were, say, actively revoking the certs of people trying to do this then it would be a different question, but it seems like the answer is the same for why anything gaming-related works worse or doesn't work at all on macOS: most gamers use Windows.
You're right. And Vulkan is based on Mantle which was already available in 2013 (in drivers; with api docs available in 2015 for some reason[1]). Why Kronos/Valve/etc didn't use Metal as a starting point, I don't know.
Metal in 2014 is a different animal than Metal in 2016.
In 2014, it assumed that the GPU has unified memory architecture, for example (i.e. the GPU can access a memory buffer that you have a pointer to). That's why it was introduced on iOS only.