The MacOS X ecosystem is 25 years old now, counting NextStep, and has an installed base of close to a hundred million systems, most of them unsophisticated personal computer users running a full Unix operating system with internet access. That is a prime target, considering there are malicious exploits that take advantage of Z-series mainframes in recent years. (one of the Pirate Bay founders got popped for looting the mainframe at a tax accounting firm)

I think it's more to do with the development culture inside Apple. The features in Swift designed to improve secure coding shows you they're actively thinking about secuirty and how to achieve it, and have been for a while.

The only regular security headlines you see about the Mac is in the Pwn2Own contest and their like, where researchers trot out vicious exploits that are then dutifully squashed by Apple in the next update, never to be seen in the wild. (And there's a reason Apple makes it a PITA to install Flash and Java these days, and includes their own very nice .pdf reader.)