Carmack has been talking about OLED for VR headsets for quite some time, and I'm sure Palmer and the rest were considering it, too. But I think it wasn't very accessible to them until recently. They're a start-up so they can't exactly go an order a few thousand 1080p OLED panels. This may have been possible now due to Samsung making the Galaxy S4 with a 1080p panel last year, and perhaps other launched some, too, and it pushed the prices down for such a panel, enough to make it viable for Oculus Rift.