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 However, it took a whole five years before D3D9 was superseded by D3D10, and subsequent releases have also been relatively slow.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.