Other folks are beating about the bush, so I'll just come out and say it:
Linux is basically irrelevant to game developers.
Other folks have been nice, but let's just be clear here. Linux's graphics story is a stupid joke, the driver story is a stupid joke, the 3D story by way of OpenGL is a stupid joke, audio is a stupid joke.
That's why it doesn't matter if you can only use DirectCX on MS platforms--because by reaching only that teensy tiny vast majority of installed computers, devs can do well.
MS has always treated their developers better than anything in the 'nix or BSD ecosystem, and that extends to better thought-out and implemented APIs.
Sorry, but that's the world we live in, and in reality there is little point in MS worrying about a non-DX API--and little point in supporting one if you're making games for PC.