Vulkan is 7 years old by now. That's the same time span between Direct3D1 (1995) and Direct3D9 (2002).
Wayland is 15 years old now btw.
7 and 15 years should be more than enough time to create stable and robust software libraries.
Direct3D versions 1 – 9 were indeed released at a rapid pace, more than one per year on average. However, it took a whole five years before D3D9 was superseded by D3D10, and subsequent releases have also been relatively slow.
Direct3D 12 is eight years old, even older than Vulkan. However, it still feels to me like a "new" API, I think because even now, many games still use DirectX 11.
I'm not sure why this is. Probably some combination of the flattening of the technology curve (hardware used to change more each year than they do today), and the fact that once a technology is more complex, it takes more time to change it, and to adopt those changes. And also, legacy compatibility becomes a concern: in 1995, we didn't have multiple decades of existing code, libraries, tooling, and complete consumer software to migrate and support.
It's because Windows.
DX9 remained on the frontline for as long as it did because Windows XP was supported from 2001 all the way to 2014 due to usage share. Similarly, DX11 remains on the frontline because Windows 7. I guess it's more apt to say they are because Windows Vista and Windows 8(.1) failed spectacularly respectively, but my point remains.
Since Windows 7 and 8(.1) are both EOL now, more games should start using DX12 Ultimate.
Deus Ex: Mankind Divided, for example, would often crash on me under dx12, until I switched to dx11. Not everyone is id Software or Epic.
Yeah, if it were goddamn Apple forcing the ecosystem to make the change. Linux is the bazaar, no one forces anything, but it also comes with disadvantages, like not adapting new tech.