It’s because there’s so few models of iPhones, the software is calibrated to use the camera so it’s much easier to add a handful of iPhones than it is for the many different other devices out there.
They also say it supports other stereo camera head mounted systems. It seems like someone out there must offer some kind of cheap headgear that you could attach a couple of GoPro cameras to, I’m sure people will be talking about this kind of thing a lot with Animator available. For example the Faceware link above requires you to pay for their software which isn’t necessary now.