On a reverse-engineered GPU like this, because of Vulkan's low-level design, implementing (early) OpenGL might actually be significantly easier.

Also, Vulkan isn't popular with game developers because availability sucks. Vulkan doesn't run on macOS. Or iOS. Or 40% of Android phones. Or Xbox. Or PlayStation. Or Nintendo Switch[1].

Unless you are targeting Windows (which has DirectX and OpenGL already), or those 60% of Android phones only, or Linux, why would you use Vulkan? On Windows, DirectX is a generally-superior alternative, and you get Xbox support basically free, and if you also support an older DirectX, much broader PC compatibility. On Android, just use OpenGL, and don't worry about separate implementations for the bifurcated Vulkan/OpenGL support. On Linux, just use Proton with an older DirectX. Whiz bang, no need for Vulkan whatsoever. Yes, some systems might perform better if you had a Vulkan over OpenGL, but is the cost worth it when you don't need it?

[1] Technically, Vulkan does exist for Nintendo Switch, but it is so slow almost no production game uses it, and it is widely considered not an option. Nintendo Switch is slow enough without Vulkan making it slower. Much easier just to use the proprietary NVIDIA library.

Because Vulkan, despite the mystical reputation it has in gaming circles, actually has fairly low adoption vs OpenGL .

Very few applications in the grand scheme of things use Vulkan, and a minority of games do.

Therefore the ROI on supporting OpenGL is very high.

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.

Keep in mind that Mesa actually implements most of OpenGL for you. Its not like you are implementing a whole OpenGL driver from scratch, you are mostly implementing a hardware abstraction layer.

My understanding is that this hardware abstraction layer for mesa is way easier to implement than a full vulkan driver, especially since the earlier versions of OpenGL only require a small subset of the features that a vulkan driver requires.