As far as I recall, Mac OS was initially proprietary project where Steve Jobs along with some other guys participated. Later, they've based Mac OS X on some portions of BSD in their NeXTSTEP, but correct me if I'm wrong, it still remains proprietary, isn't it?
"Why" is a much more complex issue. OEM distribution sure helps, but I bet there are historical reasons also. As a counter example, Asus tried to push Linux-based OS on their budget Eee PC line of netbooks, but many people formatted and reinstalled Windows there; now the entire Eee PC line uses mostly either Windows on Intel's platform and Android on ultra-portable line.
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