We suppose it would be easy to blame Sony for this, but as it turns out, it isn’t their fault and there really is nothing they can do about it. Apparently this has to do with devices needing to pass Google’s Compatibility Test Suite (CTS) which was updated during the developer preview program. In the update, it now required devices to support Vulkan or OpenGL ES 3.1 graphics, meaning that devices with Adreno 3XX GPUs would not be part of that list.
Given that the Xperia Z3 is powered by the Snapdragon 801 with the Adreno 330, it would put them flat out of luck. According to Sony developer Ola Olsson, “Even if we really wanted to give you N on the z3(c), we wouldn’t been able to do it. Not if we wanted to pass the Google CTS.” So as you can see, there really is nothing to be done.
Of course not all hope is lost as we’re sure that the enterprising Android community will find a way to create some kind of Android 7.0-based ROM for the Xperia Z3, but if you’d rather not rely on ROMs then we guess you really have no choice but to live without it, or buy a new phone.