Oculus chief architect Atman Binstock explains: "On the raw rendering costs: a traditional 1080p game at 60Hz requires 124 million shaded pixels per second. In contrast, the Rift runs at 2160×1200 at 90Hz split over dual displays, consuming 233 million pixels per second. At the default eye-target scale, the Rift's rendering requirements go much higher: around 400 million shaded pixels per second. This means that by raw rendering costs alone, a VR game will require approximately 3x the GPU power of 1080p rendering." He also points out that PC graphics can afford a fluctuating frame rate — it doesn't matter too much if it bounces between 30-60fps. The Rift has no such luxury, however.
The last requirement is more onerous: WIndows 7 SP1 or newer. Binstock says their development for OS X and Linux has been "paused" so they can focus on delivering content for Windows. They have no timeline for going back to the less popular platforms.