Previously, they were focusing on OpenGL ES 3 to get support for Desktop Linux, Windows, OSX, Android, iOS and WebGL 2.0. It seems OpenGL ES 3 has a variety of problems including poor support from hardwar vendors, buggy drivers and so it didn't really go as they had hoped.
With MoltenVK now open source allowing for Mac/iOS support, they seem confident enough to be able to work with it to drop OpenGL ES 3 and stick with Vulkan and OpenGL ES 2.0.
No word yet on when it will see Vulkan support, as they already have a roadmap ready for Godot 3.1. Once that's done, they said they will begin writing the Vulkan backend then. They are estimating the Vulkan work to take around "one or two months" and when ready they will release a new version of Godot with Vulkan (and whatever else was worked on during that time).
Good news though, hopefully we will see even more Vulkan adoption and Godot Engine too.