Because of how mesa is structured. OpenGL is notoriously terrible to implement, so there's a whole framework called Gallium that does the hard work for you, and you slot yourself into that. Meanwhile, Vulkan is easier to implement from scratch, so there's a lot less infrastructure for it in mesa, and you have to implement more of the boring paperwork correctly.

It's an accident of history more than anything else. Once the reverse engineering is further along, I expect a Vulkan driver to be written for it, and the Gallium one to be phased out in favor of Zink.