I'm not sure I agree that XCode is the reason OSX is preferred by developers. Maybe some?
Most of the developers I know prefer OSX because of the close-enough-to-linux terminal experience and compatibility of most libraries. Rails, Scala, Node, JAVA all run pretty similarly on local machines as they do on the servers.
I suppose it just depends which flavor of developer you hang with!
I would say that OSX is preferred by any power user who needs/wants access to a "desktop"-style UI, a complete filesystem experience, layered windows, etc. The bizarre opaque filesystem on iOS drives me crazy, and is one of the biggest reasons I've had trouble committing to an iPhone.
I don't know of many people working in an office environment, much less as a developer, who would be satisfied with an iPad Pro as a daily driver when it comes to getting the amount of work you can get done on something like a Macbook Air.
Perhaps I just don't understand iOS well enough. I'd love to see a video of someone actually working and multitasking effectively on an iPad at the speed that is possible on a Macbook. Most of the videos that Apple releases show work that is limited to video capture, editing and illustration.
Most of the developers I know prefer OSX because of the close-enough-to-linux terminal experience and compatibility of most libraries. Rails, Scala, Node, JAVA all run pretty similarly on local machines as they do on the servers.
I suppose it just depends which flavor of developer you hang with!