It’s *nix and THE most advanced one
Programmers need to use a lot of command line tools on daily basis like ssh, curl, build tools etc. There’s nothing like a bash shell in *nix systems(Unix, Linux. OSX is Unix-based), basically you’re left to choose between OSX and thousands of distros of Linux. (You may use CYGWin in Windows but i think it’s terrible) In my opinion, most Linux distros are still not very user-friendly (Ubuntu is probably the best you can find), OSX provides a pleasant-to-use GUI while preserving the greatness of the command-line environment. Out of the box it has everything a modern OS should have, this saves you the time needed to configure the development environment and focus on the job you need to do.
OSX is beautifully designed, slick and always