I can't help but think this is more a result of low volume and extremely tight control over their ecosystem rather than an intentionally prioritizing on security.
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.)
Second, retail-level malware is a numbers game. Malware isn't cross-platform. A malware author chooses their target based on how remunerative the target is. Windows remains more remunerative than OS X.
There is no fundamental difference between the security models of modern Windows and OS X that accounts for the disparity in malware infections.
(I'm a Mac user, and have been since ~2001.)