为什么MDCSelect:change不适用于材料设计的javascript

var selectBoxMap = {};
function initializeSelect(){
       var mdcSelectList = document.querySelectorAll(' .mdc-select');
        if(mdcSelectList){
            [].forEach.call(mdcSelectList,function(el){
                var select = new mdc.select.MDCSelect(el);
                el.setAttribute('ripple-attached', true);
                var dropDownId = $(el).find('ul.mdc-list').attr('id');
                selectBoxMap[dropDownId] = select;
            });
        }

}


$(async function(){

  var results = await $.get('https://jsonplaceholder.typicode.com/users');
var usersStr = '';
for(var i = 0; i < results.length; i++){
    var item = results[i];
           usersStr += `<li role="option">
                              <span></span>
                              <span>
                                  ${item.name}
                              </span>
                          </li>`
}

initializeSelect();

$('#select_dropdown').html(usersStr);

     for(var key in selectBoxMap){
            if(selectBoxMap[key]){
              selectBoxMap[key].listen('MDCSelect:change', (e) => {
                  alert('dropdown changed');
              });
            }
        }

});