Due to the limitations on mobile devices. Currently they have few hardware buttons, touch screen, microphone and maybe camera. Right now the options are bounded

Due to the limitations on mobile devices. Currently they have few hardware buttons, touch screen, microphone and maybe camera. Right now the options are bounded by these limitations. Fingerprint or iris recognition would require additional hardware. Most of the customer probably would not be willing to pay extra for these. Also they might be difficult to implement well on mobile device. And the unlocking must be very easy to use and reliable.