These days, OSX is simply a better product than is Windows (stability, security, feature set). OSX also has things like iTunes (largest provider of digital content world-wide... besides Pirate Bay), FinalCut, Logic, Aperture, and GNU/BSD tools. That's a hard line up for Windows to beat. More importantly, I think, is that OSX and iOS work well with one another. If I have an iPad, an iPhone, and an iMac my content syncs easily and transparently. When taken with iTunes this is simply tough to compete with.

tl:dr digital content delivery made Apple king.