Probably because until relatively recently, MS has not open sourced very much or any software, whereas Apple has a number of notable open source projects (LLVM/Clang, Webkit/JSCore, ...). OS X is also build on open source software (BSD, Mach) whereas Windows is not.

There are other reasons too, but those are the biggest ones, I think.

you forgot CUPS, the thing that lets most *nix platforms print.

An outsider's view: Microsoft has long relied on their open-if-you-have-money model (shared source), wherein if you mattered to them, you could pay them money to see the source. Presumably, if you paid them more you might even be able to make changes and use them for your own purposes.

Apple, OTOH, embraced open-source from the beginning (for a given value of embrace). Lots of open-source projects that go fine as long as you do it the Apple way and don't mind them passive-aggressively slowly pulling away, but not enough to kill the project.

I love me some Apple, but let us not pretend that they're saints compared to MS when it comes to actual, existing projects.