Why does $page['content']['#attached']['js'] works , while $page['#attached']['js'] does not?
For #attached elements to be processed, the array needs to be run through drupal_render(). That never happens for the $page array.

Parts of it are rendered, e.g. in page.tpl.php:

&lt;?php print render($page['content']); ?&gt;


But never the $page array itself.