When Valve released Portal 2 in 2011, it was still 32-bit. This wasn't a problem for Windows users and still isn't. Valve tried as much with the Windows release of that game as the Mac one.

Mac is just an unstable platform. Developers either put up with it or get shown the door. Valve doesn't like to be pushed around by the platforms it uses, which is why they rejected the PS3 and began to prop up Linux as a viable alternative for gaming.