The news that Overwatch wouldn't be making the journey to Mac was revealed during Blizzcon, and while some weren't surprised others were a bit confused as Blizzard generally supports Mac gaming. According to game director Jeff Kaplan, it's because of Apple's policies with OSX.
"It was a result of not having all the technological support we needed to make the game viable on Mac systems," said Kaplan. "We have a real love and dedication for Mac players, they’ve been extremely loyal to us and we love giving them Blizzard games. But when dealing with the PC, the Xbox One and the PS4 - all of which are extremely welcoming to the technological needs to run a next-generation shooter - in a lot of ways we felt left behind, that we weren’t given the support we needed to make a great product on the Mac."
OSX does have some graphics technology that compared to Windows seems pretty underpowered. While optimised for Mac hardware, many do choose to dual-boot into Windows to run games on their machines. There's also the issue of driver support from companies like AMD and Nvidia, which again isn't up to the level that Windows PCs currently get.
Kaplan thinks this could change in future, but only if Apple put more effort into making it worthwhile for companies to develop games for Mac.
"I think Apple’s an amazing company, and I hope, I really hope Mac players are vocal back to Apple that Mac gaming is extremely important and should be extremely important. I can only guess how important things like iPhone and other products are to a company like that - but I think Mac gaming should have equal importance."