Firstly the team would need to rebuild the client to support a completely new OS (OSX) which uses a different graphical system (they couldn't use DX on a Mac), every time there was an update to the .exe they would need to do the same update to the mac client, then there is QC, at the moment there are a lot of people who can make sure there are no exploits/issues with the Windows client, they wouldn't have the same userbase for OSX. Then there is funding, I don't really want my subscription fee to be used to develop the client for a niche system which less than 10% of the users actually own when that 10% can already run the game if they install Windows.

PS3/4 clients may not own a PC, or may not own a PC capable of playing XIV, Mac owners who want a Mac client should be able to run XIV so there is no reason on earth for it to be developed for the Mac, you haven't actually given a good reason as to why resources should be put aside for a Mac client, why 90+% of the userbase should have their subscription fee used to develop it.