why $(window).scroll() need inside deviceready event handler?

I'm writing Cordova project with jQuery Mobile.I have the code to detect user scroll to the bottom.It's work but I don't know why the code need to be placed inside deviceready event handler? Waiting for something get ready?

function onDeviceReady() { $(window).scroll(function () { if ($(window).scrollTop() + $(window).height() == $(document).height()) { alert("bottom!"); } }); }