Not able to lazily update a controller scope with a REST service - Angularjs

I have basically created a cache in my Angular service. If the REST call has already been made, the an array is returned, else it makes the REST call to get the needed info. Here is my service:

define(function(require) {

var module = require('portal/js/services/app.services');

return module.factory('providerService', function($resource) {
  return {
    requestingProviders: [],
    getRequestingProviders: function() {
      var that = this;
      if(this.requestingProviders.length === 0) {
        this.getResource().search({
          role: 'REQUESTING_PROVIDER'
        }, function(data) {
          that.requestingProviders = data.providers;
          return that.requestingProviders;
        });
      } else {
        return that.requestingProviders;
      }
    },
    getResource: function() {
      return $resource('/api/v1/providers/:providerId', {providerId:'@id'}, {
        search: {
          method: 'GET',
          headers: {
            'RemoteUser': 'jhornsby',
            'Content-Type': 'application/json'
          },
          params: {
            limit: 2000,
            organizationId : '0001194'
          }
        }
      });
    }
  };
};

Here is an example of me accessing the service from a controller:

define(function(require) {
  var module = require('portal/js/controllers/app.controller');

  module.controller('AddPatientCtrl', function($scope, providerService) {
   $scope.providers = providerService.getRequestingProviders();
  };

  return module;
});

When the REST call returns, it does not update my scope.providers variable in my controller. When I try to access it a 2nd time, then it works fine by accessing the array instead of calling out to the REST service. WHy won't my scope update in my controller during the REST call?