[orclev]

Because game developers mostly don't pick the backend, game engine developers do. The vast majority of game developers pick a game engine and that drives most of their other technical decisions. There are really only a dozen game engines that have enough market share to matter, and a decent chunk of the biggest ones were built on top of DX for various reasons. OpenGL, while a great concept, was a fairly flawed execution for quite a while (its gotten a lot better in the last 10 years or so), so I can at least partially understand why in the past someone who doesn't care at all about cross-platform support might have steered clear of it.