Largely, it comes down to Android being a bad OS for input latency (indeed, Oculus and Samsung had to perform what John Carmack described as "deep hacks" into t

Largely, it comes down to Android being a bad OS for input latency (indeed, Oculus and Samsung had to perform what John Carmack described as "deep hacks" into the Android kernel to get it to a reasonable state with Gear VR), most phones not having a high-enough quality IMU for tracking head motion, and most phones not having a low persistence display (high persistence causes a smearing effect in the visual). So while "Cardboard" can "work" on most phones, it's a pretty terrible experience, once you get past the awe factor of seeing stereoscopic visuals with head tracking. Us developers have largely stopped making anything with basic "Cardboard" support, for fear of giving people a bad first impression of VR. As for Daydream, a lot of the point of the Daydream system was to get around those issues I mentioned. First, it required phones to meet a certain hardware profile to before they were allowed to market themselves as "Daydream Compatible". Second, it included changes to the operating system to provide much lower latency in input tracking and change display settings to reduce pixel persistence.