Google did not give enough support to Motorola
According to Green Bot, Google impeded efforts to create the ultimate Motorola Android phone that is tightly integrated with all services of the company from Mountain View. While Motorola has been involved in work on contactless voice commands in Moto X, since Google did not have enough support.
Motorola also wanted to be closer to YouTube and Google+ teams of Google, to build closer integration of these services in Moto X, but again to no avail. Also, Google have played a minor role in the patent battle between Samsung and Motorola.
These actions on the part of Google may have been motivated by the fact that if allowed to create the ultimate Android smartphone, the other partner companies would be resisted. And Google has always claimed he did not want to focus all efforts into one hardware manufacturer.