OpenGL-on-Vulkan compat layers aren't magic. For them to support a given OpenGL feature, an equivalent feature must be supported by the Vulkan driver (often as an extension). That means you can't just implement a baseline Vulkan driver and get OGL 4.6 support for free, you must put in the work to implement all the OGL 4.6 features in your Vulkan driver if you want MESA to translate OGL 4.6 to Vulkan for you.

Plus, this isn't Alyssa's first reverse engineering + OpenGL driver project. I don't know the details but I'd imagine it's much easier and quicker to implement a driver for an API you're used to making drivers for, than to implement a driver for an API you aren't.

They started with targeting older OpenGL to get a basic feature set working first. I guess from there, getting up to a more recent OpenGL was less work than doing a complete Vulkan implementation, and they probably learned a lot about what they'll need to do for Vulkan.

Ok, this makes a lot of sense—OpenGL sort of forms a pathway of incremental support.

Along with that, it's more immediately useful as it's used for desktops and compositers still, so getting a useful environment necessitates it.

I thought something similar, but from their comments, to support OpenGL over Vulkan you need higher versions of Vulkan anyway and it's still a big effort. So they decided to go with (lower versions of) OpenGL first to get something functional sooner.