The FaceTime camera is primarily used for video conferencing (which is why it's called FaceTime). Good luck finding a widely used platform that does 1080p at 30fps with DECENT latency. Facetime, Hangouts, etc. usually transmit at 720p to keep the latency under 150ms. It is ideal to keep that latency down for a better/fluid than to transmit a 1080p feed which stutters/lags. I'm pretty sure Skype can do 1080p, but then again, all parties must be under ideal network connections in order to have a decent experience, which most of the time doesn't happen.
One can argue for those who do Twitch any other broadcasting medium, but most broadcasters have a dedicated web camera anyway to provide the best feed quality (webcams are always better than the laptop) while still being able to manipulate camera angles and what not (without having a fixed point of view like a laptop camera).
I guess the one and only type of users who would really benefit the 1080p camera would be selfie junkies (don't they use their cell phones anyway?) or Photo Booth users in which... well. yeah.
Apple does their market research. They will outweigh the benefits as well as drawbacks of each and every component that goes into their products. A 720p is perfectly fine for a laptop webcam and will be for a very long time until a better network infrastructure is widely available to take advantage of a higher resolution camera and the large bandwidth it requires.
