Apparently, Google and Huawei have made unlocking the bootloader on the Nexus 6P a questionable proposition. They built a fuse into the Nexus 6P called QFuse, and whenever someone unlocks the bootloader, it blows this hardware fuse permanently. Once blown, there is no way to reverse it back to its "locked" default setting.
This is similar to what Samsung did with Samsung Knox. Before you start hollering that they hate developers, supposedly the primary reason for the QFuse is simply to enhance the security of the device. It is meant to make Android Pay more secure and help let you know if the device has been tampered with. Of course, that is small comfort for folks who want to tinker with or develop on a completely unlocked Nexus 6P.
We can only cross our fingers that Google will release an unlockable Developer Edition that eschews the QFuse. Sound off and let us know if this affects your perspective on Google's latest Nexus device. Here's our dedicated Nexus 6P section for further discussion: Nexus 6P by Huawei | Android Forum at DroidForums.net