Why I Can't Use AttachEvent With InnerHTML;

if i use AttachEvent like this,it can't work;
eg:

var img = document.createElement("img");
img.attachEvent("onclick",alert("test"));
var div = document.createElement("div");
div.appendChild(img); //can't work;
div.innerHTML="<-click this";

but i use attachEvent like this,it work; eg:

var img = document.createElement("img");
img.attachEvent("onclick",alert("test"));
var text = document.createElement("span");
text.innerHTML="<-click this";
var div = documet.createElement("div");

div.appendChild(img); //can work
div.appendChild(text);