Most 3D-sensing platforms — like the PrimeSense chip inside Microsoft’s original Kinect — have a comparatively enormous power draw, usually over 1 watt. That’s orders of magnitude higher than what’s needed in order to make it a viable option for use in mobile devices, where power is always at a premium. The iPhone’s battery hovers around 1,500 mAh, which is many, many times smaller than is needed to power such a chip for any length of time. The Myriad 1 operates in the range of a couple hundred milliwats — making putting this kind of chip on a phone possible.
It's hard to understate the importance of battery efficiency in mobile. Battery life is often the bottleneck for new technologies -- the first 4G LTE phones, for example, were battery killers -- and that's not going to change given that radical new battery technology is years away.
The battery problem will become even bigger with the push into wearable technologies. Google Glass struggles to last through a day of moderate use, and with the camera in use it doesn't last an hour. We've yet to see a smartwatch that can last a full week.
Although Project Tango exists in phone form today, it seems much more practical as a technology for wearable devices. If I'm walking down the street in search of a store or restaurant, I'm not going to start waving my phone around to get descriptions of each storefront. But let's say I'm wearing something like Google Glass. Now it makes more sense to have an app that knows what I'm looking at, interprets the 3D position of the building and overlays a description in 3D space.
Another example: If I'm using a smartwatch, perhaps it could detect whether I'm home alone or at a friend's house with a half-dozen people, and re-prioritize which notifications I get.
By comparison, the spatial awareness that Project Tango provides will be lost on smartphones, which stay in our pockets until we know we need them. If the claims about power efficiency by Google and partner company Movidius hold up, Project Tango isn't going to matter much for smartphones. It's really a play for what comes next.