Why does `refButton` get `null` in the following JavaScript code?

```
<html>
<head>
    <title></title>
    <script type="text/javascript">
        var refButton = document.getElementById("btnButton");

        refButton.onclick = function() {
            alert('I am clicked!');
        };
    </script>
</head>
<body>
    <form id="form1">
    <div>
        <input id="btnButton" type="button" value="Click me"/>
    </div>
    </form>
</body>
</html>

```