Gaming in the 90's started out on OpenGL, which was and is cross platform. Ports for Quake were being made for things like SGI workstations, because it was relatively straightforward. Microsoft headed off further development in a cross-platform gaming environment by creating a Windows-only API, and making sure developers used it, with lots and lots of money. Windows became the de facto platform because Microsoft used their monopoly to kill off the burgeoning threat of an open ecosystem. If it hadn't done this, Macs and Linux could have been viable gaming platforms, despite their relative marketshares.