As others have mentioned, on OS X (and Linux), OpenGL is the only game in town for hardware-accelerated graphics. So the question really comes down to: why do developers use Direct3D instead of OpenGL on Windows?
One possible reason, as suggested in the comments, is that they started out as a Windows-only project and later decided to add OS X / Linux support, but didn't want to throw out their already-working Windows code. But another possible reason is that the developers may simply consider Direct3D to be superior to OpenGL, enough that they would rather write their rendering code twice than use OpenGL on Windows.
Part of this is clearly a "holy war" kind of issue - Direct3D and OpenGL use very different styles of interface, have different design values, and appeal to different coders' tastes. Like Mac vs PC, or Emacs vs vim, or tabs vs spaces, one can endlessly debate these things without ever convincing anyone.
Putting all that aside, though, I think it's fair to say that during the last several years, Direct3D has been objectively superior to OpenGL for Windows graphics programming, for the following reasons:
- For a good while, OpenGL seriously lagged behind in features. Direct3D 10 and 11 added support for new GPU features like geometry shaders, tessellation and compute shaders that it took OpenGL years to catch up to. There's a good Tom's Hardware article from a few years ago that gives you a taste of how disappointed people were about the lack of OpenGL feature development during that time.
- Direct3D has historically had much better tools: the debug runtime / debug layer, which generates warning messages about incorrect or questionable API calls you make; and PIX, the graphics profiler/debugger. For a long time, OpenGL had nothing comparable.
- The major GPU vendors have usually made Direct3D driver support a higher priority than OpenGL driver support. As a result, bug fixes and performance improvements often come sooner to the Direct3D drivers than the OpenGL ones.
- Console compatibility: from what I understand (not having worked with it myself), the XBox 360 graphics API is very similar, if not identical, to Direct3D 9. To whatever extent this is true, it likely means an easier time of porting games between PC and 360. It also means that if you're shipping your game on the 360, you have to write a Direct3D renderer anyway - so you might as well use it on Windows too.
By the way, our own Nicol Bolas also wrote an answer to a related question at programmers.se that goes into a lot more detail about the history of the OpenGL/Direct3D rivalry.
All that being said, there are signs that the tide is starting to turn. OpenGL developers have been working very hard to rectify the features and tools issues. OpenGL has now reached feature parity with Direct3D 11 and even gone a bit beyond. Timothy Lottes of NVIDIA wrote a blog post outlining his reasons for switching to OpenGL, with a lot of links about the new extensions and tools. Valve Software recently reported that they got an OpenGL version of Left 4 Dead 2 working and it was (slightly) faster than the Direct3D version, indicating that the driver support is there. So, it's possible we will see a swing back toward multi-platform developers using OpenGL on Windows. Or we may not.