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/

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.