The majority of people who make a gaming machine will put Windows on it. OSX and Linux have done well at getting more developers to make their games compatible

The majority of people who make a gaming machine will put Windows on it. OSX and Linux have done well at getting more developers to make their games compatible with them, but I can't even name any games that work on either of those and not Windows, except the ones that are free in the repositories. Kickstarter'd games all pledge Windows from the start and may have Linux and/or OSX added on if they get enough support. They definitely still own the PC gaming space.