问为什么这会到达main()，而jQuery仍然没有定义？

EN

!function loader (i) {
        var requires = [ 'http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js'
                       , 'https://ajax.googleapis.com/ajax/libs/jqueryui/1.8/jquery-ui.min.js'
                       ];
        var loadScript = function (i) {
                    var script    = document.createElement('script');
                    script.type   = 'text/javascript';
                    script.src    = requires[i];
                    script.onload = (++i === requires.length) ? function () { loader(i); }
                                                              : function () { main(); }
                    document.head.appendChild(script);
            };

        loadScript(i || 0);
}();

function main () {
    console.log(jQuery.version);
    console.log(jQuery.ui.version);
}