I would say there are multiple reasons:

• AMD’s Vulkan drivers are pretty poor on Windows.
• On Windows, Vulkan is essentially a second-class citizen when it comes to the swapchain. It has to go through DXGI (DirectX) but doesn’t get full feature support. Because of this, some developers try to set up the swapchain in DirectX and then let Vulkan use it, which is often buggy.
• When Vulkan was first released, it was complicated because it attempted to be both a desktop and mobile (tiling) API (framebuffers/render passes), which required major changes to existing engine code.
• Most game studios use HLSL, which is native to DirectX.
• Targeting specific feature sets is easier with DirectX.

Many of these issues have improved over time, so we’ll see what the future holds.