There's a major difference between using it on a laptop hybrid where it's mostly stationary, in a fixed position and your naturally looking directly at it - and the time it takes to unlock isn't quite so important.
On a mobile phone the device behaves different, as does your behaviour / usage of it.
Those seconds it takes to take your phone out of your pocket, raise it to your face, wake device, let it scan and hope it recognises you or then have to input passcode if it fails - is exponentially far longer than a fingerprint scanner which can wake your device and unlock it in the time it takes to simply take it out of your pocket.
The practicalities change dependent on device. On a mobile sometimes you want to v. quickly unlock without actually having to gaze upon the device (such as in a car or such) whereas with the iris scanning that again is simply not practical.
A case where the technology doesn't suit the device.
