The video background option is disabled on mobile by default due to various incompatibility reasons.
Videos will not autoplay on Android / iOS. The restriction is implemented at the operating system level to protect device owners from having their download allowance consumed by the content they may not be interested in.
You can read a thread on stackoverflow.com.
So, we have added some advanced optimization methods and have provided a provision for users to input a placeholder image or auto add from youtibe which is displayed when your site run from Mobile device.