Mac runs off a custom Linux kernel that is closed off (you have to buy the dev kits from Apple and buy the yearly dev support). You also have to port the code over from Windows to Mac (no easy task by any means) on top of "low" interest Mac users have for using their machines for gaming.

From a business standpoint, the man hours it would take, supporting the code structure of another OS (we already have Windows, PS3, and the PS4), Adding driver support, getting approval from Apple, and writing ports for ALL their patches wouldn't make sense. It just costs too much money.

Don't get me wrong, I'd love to have a native Mac client for XIV, but the reality is, it just costs too much money to make it feasible.

There are alternatives however:

Bootcamp - Dual boot into Windows (this only works due to Mac chipsets being Intel. For the time being. There is talk they are reverting back to their custom Ax processors)

Wineskin Wrappers - http://appdb.winehq.org/objectManager.php?sClass=version&iId=28853 (pain in the ass to configure but works)

Already Configured Wrapper - http://lodestoneradio.com/home/article/1716556/m/7398449 (it takes a small donation, but it works pretty well. I use it at times when I'm on my 2010 MacBook Pro. This, to me, is the best option available right now)