I'd imagine it was really popular among console developers, who would usually develop two versions of their game anyways (a DirectX one for Xbox/PC and an agnostic one for Mac/Playstation/other). DirectX is traditionally considered easier to use (and comes with PC tooling) so I could see how people would prefer it for debugging.
Sony only supported it on PS 2, and quickly moved into LibGNM(X) as almost no one cared, Nintendo had a OpenGL like API on the Wii, and while the Switch supports GL 4.6/Vulkan, its main API is NVN.
Obviously, lots of games only had an OpenGL backend. So those devs simply didn't have access to those tools. It's the one outlier API with bad tooling.
The main problem with Khronos APIs has always been "go fishing" attitude, Vulkan isn't much better, even with the LunarG SDK.
Yeah, except nobody did this. The cutting-edge features came to Direct3D first because Microsoft had early access to what the GPU vendors were doing and could plan out what that would look like from an API perspective. The vendor extensions only came out around the same time as the DirectX support, maybe later. Core OpenGL support would onlu emerge years later.
> As a game developer it's invaluable to be able to swap over to a Direct3D backend for debugging even if you end up using OpenGL as your default.
What? Name a game developer who's done this in the past 20 years -- use Direct3D for debugging and OpenGL as the default. Only Id Software actually used OpenGL and Vulkan seriously, and they are owned by Microsoft now so that will change.