Because Vulcan is very much like the APIs you use on console development to talk to the GPU. With the main difference that it's been designed to be GPU vendor neutral.

Console vendors have always designed their own APIs why change now?

I would imagine it comes down to costs. If this api does a good job, why spend the effort (read: money) to create a new one with all of the tooling around it?

Though, I should note I doubt this will really save much. Just, on paper it is a costs savings. (Unless, as usual, I'm wrong on something.)

Consoles tend to have custom hardware architectures that don't map at all to generic APIs.

I think that was a more true statement before MS enterred the game.

Also, for something as low level as this api sounds, it probably mapped decently, while still requiring another set of primitives for processor and memory control.

That is, I think the point of this api is to be a subset that can remain a bit more common between platforms. Not the all inclusive api that rules everything.