Why can't the $rootScope be accessed in the template of a directive with isolate scope?

With isolate scope the template of the directive does not seem to be able to access the controller ('Ctrl') $rootScope variable which, however, does appear in the controller of the directive. I understand why the controller ('Ctrl') $scope variable isn't visible in the isolate scope.

HTML:

<div ng-app="app">
    <div ng-controller="Ctrl">
        <my-template></my-template>
    </div>
    
    <script type="text/ng-template" id="my-template.html">
        <label ng-click="test(blah)">Click</label>
    </script>
</div>

JavaScript:

angular.module('app', [])
    .controller('Ctrl', function Ctrl1($scope,  $rootScope) {
        $rootScope.blah = 'Hello';
        $scope.yah = 'World'
    })
    .directive('myTemplate', function() {
        return {
            restrict: 'E',
            templateUrl: 'my-template.html',
            scope: {},
            controller: ["$scope", "$rootScope", function($scope, $rootScope) {
                console.log($rootScope.blah);
                console.log($scope.yah);,
                
                $scope.test = function(arg) {
                    console.log(arg);
                }
            }]
        };
    });

JSFiddle

The variable is accessed with no isolate scope - as can be seen by commenting the isolate scope line:

        // scope: {},

You can try this way out using $root.blah

Working Code

html

 <label ng-click="test($root.blah)">Click</label>

javascript

  angular.module('app', [])
    .controller('Ctrl', function Ctrl1($scope,  $rootScope) {
        $rootScope.blah = 'Hello';
        $scope.yah = 'World'
    })
    .directive('myTemplate', function() {
        return {
            restrict: 'E',
            templateUrl: 'my-template.html',
            scope: {},
            controller: ["$scope", "$rootScope", function($scope, $rootScope) {
                console.log($rootScope.blah);
                console.log($scope.yah);
                
                $scope.test = function(arg) {
                    console.log(arg);
                }
            }]
        };
    });

Generally, you should avoid using $rootScope to store values you need to share between controllers and directives. It's like using globals in JS. Use a service instead:

A constant (or value ... use is similar):

.constant('blah', 'blah')

https://docs.angularjs.org/api/ng/type/angular.Module

A factory (or service or provider):

.factory('BlahFactory', function() {
    var blah = {
        value: 'blah'
    };
    
    blah.setValue = function(val) {
      this.value = val;
    };
    
    blah.getValue = function() {
        return this.value;
    };
    
    return blah;
})

Here is a fork of your Fiddle demonstrating how you might use either

1. Because of the isolate scope $scope in your controller Ctrl and in the directive controller don't refer to the same scope - let's says we have scope1 in Ctrl and scope2 in directive.

2. Because of the isolate scope scope2 do not prototypicallly inherit from $rootScope ; so if you define $rootScope.blah there is no chance you can see it in scope2.

3. What you can access in your directive template is scope2

If I sum it up, here is the inheritance schema

    _______|______
    |            |
    V            V
$rootScope     scope2
    |
    V
  scope1


$rootScope.blah
> "Hello"
scope1.blah
> "Hello"
scope2.blah
> undefined

I know this an old question. But it didn't satisfy my inquisition about why the isolated scope won't be able to access properties in the $rootscope.

So I dug in the angular lib and found -

$new: function(isolate) {
  var ChildScope,
      child;

  if (isolate) {
    child = new Scope();
    child.$root = this.$root;
    child.$$asyncQueue = this.$$asyncQueue;
    child.$$postDigestQueue = this.$$postDigestQueue;
  } else {

    if (!this.$$childScopeClass) {
      this.$$childScopeClass = function() {
        // blah blah...
      };
      this.$$childScopeClass.prototype = this;
    }
    child = new this.$$childScopeClass();
  }

This is the function called by angular whenever a new scope is created. Here it's clear that any isolated scope is not prototypically inheriting the rootscope. rather only the rootscope is added as a property '$root' in the new scope. So we can only access the properties of rootscope from the $root property in the new isolated scope.