Are there any Device Administrators (like corporate e-mail policies, etc.) installed? Or if you have a carrier version, have you made sure that the feature isn't disabled for your specific carrier model? You can try going into the App Manager (show system apps) and clear data on the Settings (this would probably also default everything else in your settings, though) and see if it becomes available; it's worked for a few where they got some features disabled after a Device Administrator removed them.