Hey Daisuke,
We have actually did this on purpose in 2.1 and the main reason being that the screen on mobile devices are already so small, displaying a custom background really wouldn't make sense since at least 85% of the background is covered by the form
If you really need this, what you can do is to edit the file /administrator/components/com_easysocial/includes/login/login.php and at line 52, locate the codes below,
[gist]
$image = "background-image: url('" . $image . "');";
[gist]
Replace it with,
[gist]
$image = "background-image: url('" . $image . "') !important;";
[/gist]
By the way, is the background very important for your mobile site? If so, may I know why?