We don't know, could be a number of things. Likely culprits are a heat issues, screen resolution, OR framerate.
I'm in a similar boat with my Mate 9. I feel it should be plenty good enough for Daydream, BUT apparently Huawei and Google don't share my views. Maybe at some point a "hack" will be found that enables phones that are close to use it. A good example is the Nexus 6P. Google says no, BUT it's known for a fact that they do/did some daydream testing on that device so it certainly CAN work. So who knows what the future holds.