Because good enough IMUs and optical systems for positional tracking are hella expensive compared to where in the market this thing is targeted.
There'd need to be either:
- a very expensive IMU in the controller, talking $300-500
- a camera and hefty CPU in the controller,
- a camera and dedicated image processing CPU on the headset
Vive positional tracking works because you control the environment. Oculus positional tracking works because you're already running on a hefty PC. You don't have either of those advantages on mobile, so you're left with either IMU sensor integration or SLAM, both of which are more expensive than $100 to do well and leave processing time for graphics.
The Gear VR/Oculus IMU is only good for orientation. Positional tracking with the Oculus Rift is done with an external camera, detecting a constellation of infrared LEDs on the headset itself. Gear VR thus does not have positional tracking.
To be able to integrate the acceleration data from an IMU to get positional data out of it is not something you can do with commodity IMUs, even the one made for Oculus. They're too imprecise and too inaccurate and too slow, and any analysis tricks to improve the results all introduce latency. Oculus' sensor has very little drift (though it still has drift, which is another thing the camera system on the desktop Rift can correct for) and the noise in the orientation data is not noticeable or can be filtered much more simply without introducing "too much" latency.
There are IMUs that are good enough to integrate acceleration data to position data, but they are prohibitively expensive for this application.
For what it's worth, I used to see a marked difference in the quality of orientation tracking between my Galaxy Note 4 in Google Cardboard vs. the Gear VR, but I see no different between GC and GVR on my Galaxy S7.
There'd need to be either:
Vive positional tracking works because you control the environment. Oculus positional tracking works because you're already running on a hefty PC. You don't have either of those advantages on mobile, so you're left with either IMU sensor integration or SLAM, both of which are more expensive than $100 to do well and leave processing time for graphics.