Angularjs $state couldn't navigate back

I can only navigate from page 1 to page 2 but couldn't get back from page 2 to page 1.

demo http://codepen.io/anon/pen/qEOJBp

I wonder why? The code look flawless for me :

<ion-nav-view name="home"></ion-nav-view>
<ion-nav-view name="threadContent"></ion-nav-view>

<script type="text/ng-template" id="home.html">
  <ion-view name="home">
    <ion-content>
      <h2>Home Page</h2>
      <p>Here is the main route for the app.</p>
      <a href="#/threadContent">GO</a>
    </ion-content>
  </ion-view>
</script>

<script type="text/ng-template" id="threadContent.html">
  <ion-view  name="threadContent" title="Thread Content">
    <ion-content>
    <a href="#/home">Back</a>
       <h2>Using the app</h2>
       <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Facilis architecto hic officia quasi excepturi sequi deleniti maiores consectetur veritatis sint?</p>
    </ion-content>
  </ion-view>
</script>

$rootScope.$on('$stateChangeSuccess', function (event, to, toParams, from, fromParams) {
    $rootScope.$previousState = from;
    $rootScope.$previousStateParams = fromParams;
});

On my project I have used ui.router, but main idea is: Catch change state event and save data to root scope. And you will always have an ability back to previous sate.