Using as XML

[onenterflash]

I want to know why I can't use this code

var xml:XML = new XML();

var loader:URLLoader = new URLLoader();

function xmlloaded(e:Event):void {
    
    xml=e.target.data as XML;
        
        trace(xml.toString());

}

loader.load(new URLRequest('photogallery.xml'));

loader.addEventListener(Event.COMPLETE,xmlloaded); 

I know the problem is

xml=e.target.data as XML;

but I want to know why I can't use the as XML instead of XML(e.target.data)?

thank you