"Why can't I turn off my X1 TV Box with my remote control power button? This feature was disabled to address issues that our customers had with keeping their TV and TV Box power synchronized. Because the remote communicates with the TV and TV Box using different signaling technologies, one device would sometimes get turned off while the other was turned on. We do realize that disabling the Off button on the remote control isn't ideal, and we are looking into options to address this issue. In the meantime, you can turn the TV Box off by saying "power down" into your Voice Remote."

https://www.xfinity.com/support/articles/x1-power-save-faqs