Bootstrap modal event is only captured when jQuery.on is used

I am in the process of removing Bootstrap 3 and jQuery from an application. In doing this I came across a situation where an event works perfectly fine with jQuery, however, when I try to use vanilla JavaScript to do the same thing the event is not triggered at all.

Here is a working snippet, look at the jQuery function where it does an alert('here') and after you run this you can see it working:

 $("#myModal").on("hidden.bs.modal", function (e) {
  alert("here");
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.min.js"></script>
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css" rel="stylesheet"/>
<!-- Button trigger modal -->
<p>
 Click the "x" icon in the modal and it should trigger an alert from the jQuery function below...
</p>
<button type="button" id="mymodal" class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal">
  Launch modal
</button>

<!-- Modal -->
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
        <h4 class="modal-title" id="myModalLabel">Modal title</h4>
      </div>
      <div class="modal-body">
        ...
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div>
  </div>
</div>

However when I try to convert that same jQuery to JavaScript like this it does not trigger the alert:

document.getElementById('myModal').addEventListener('hidden.bs.modal', function(e) { 
  alert('why dont I alert');
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.min.js"></script>
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css" rel="stylesheet"/>
<!-- Button trigger modal -->
<p>
 Click the "x" icon in the modal and it should trigger an alert from the jQuery function below...
</p>
<button type="button" id="mymodal" class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal">
  Launch modal
</button>

<!-- Modal -->
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
        <h4 class="modal-title" id="myModalLabel">Modal title</h4>
      </div>
      <div class="modal-body">
        ...
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div>
  </div>
</div>

I am curious why the second example does not work when the first one does – if I throw a CustomEvent with hidden.bs.modal myself it works just fine, but still doesn’t answer the question of why this example does not work.