Earlier quoted context omitted.
Valve's problem with Windows is that they perceive them to be going after a more closed system. Snubbing Windows for the far more closed OSX would be ridiculous, so that's never going to happen. The Linux market for games is absolutely tiny compared with Windows, so there's no way they are going to release a major title on Linux first either.
OS X is not "far more closed". Darwin, the UNIX kernel and utils that OS X runs on top of is fully free and open-source. It's just the Apple-specific APIs like Cocoa, Carbon, Quartz, Aqua and all that jazz that is closed-source and proprietary. Can I get source access to the NT kernel on Windows? Nope. That's Microsoft's secret and novel invention that is supposedly the best OS in the world, so they have to keep it o…
OS X is specifically designed not to work non-Apple hardware, even hardware that is perfectly capable of running it.
Parts of it are open source, mostly because those parts were an extention of existing open source software.
"OSX" is not free in any sense of the word.
Microsoft are still a far cry from the likes of GNU/Linux in terms of openness, but almost nobody would describe the Mac platform as "more open" than Windows.