The Nexus 6 comes with Android 5.0 Lollipop, Google's latest mobile operating system that brings a number of cool new features to any device running it. However it appears that on the Nexus 6 Google has disabled the tap-to-wake function.
This feature would have allowed Nexus 6 owners to double tap the screen to wake the device up rather than having to press the power button.
The Nexus 6 initially had the feature, however in a code commit on Google's website it appears that Google has disabled the feature.
There hasn't been an official explanation about why Google has taken the feature out of the Nexus 6. The Nexus 9 includes the feature without any problems.
A potential reason for Google disabling the tap to wake feature was mentioned by PhoneArena. As the Nexus 6 is manufactured by Motorola it comes with an Ambient Display, which is similar to the active display found in the Moto X.
Motorola's unique display might be causing issues with the tap-to-wake feature, but until it's officially confirmed this is all speculation. What we do know is that sadly the Nexus 6 will be shipping with one less feature.