There are many developers who value high several OS X features, such as well-implemented HiDPI mode with custom resolutions, super-easy backup solution, solid energy saving & suspend & resume functionalities (no conflicts with crappy 3D drivers), etc, etc. Those features wouldn't mean much without the possibility to easily¹ run FOSS on unix-like system.

¹) The differences between various package management systems are not significant in this context.