Here's at least one reason why: Check out the 'Star Swarm' stress test comparing the Oxide Games' Nitrous engine demo on AMD's Mantle versus Microsoft's DirectX 11. Nothing less than a generational leap in graphics tech for desktop gamers comparing tech vs. tech in a graphics app that's built from the ground up to use Mantle vs. DirectX 11 that's merely 'compliant' with it. See the video:
(Ghastly silence from the legion of PC gamers who mocked the new MacPro's dual AMD D700 graphics cards rather than going with NVidia; "LOL! Those aren't GAMER cards, they're WORKSTATION cards!")
What is it going to mean for Microsoft when they don't even rule PC gaming market share anymore?
This is a company getting nibbled to pieces by ever-increasing numbers of piranha.
Microsoft doesn’t have to worry about losing PC gamers to Apple, after all AMD/ATI cards fit in PCs just fine. Microsoft will rule the PC gamer market until game developers start supporting Linux.