I don’t know why it’s disabled by default and I use it quite often because I’m always playing around and installing things so it’s incredibly irritating to me that you can’t just press ctrl+alt+backspace to restart x.
To enable it you can do it through editing the xorg.conf file:
1. Open a terminal and type sudo gedit /etc/X11/xorg.conf
2. Add this section to the file:
Option "DontZap" "false"
3. Save and restart.