Question:

Why is it easily possible to theme node add forms and not node edit forms?

Beau: 2 weeks ago

Why can i override:

page--node--add--mycontenttype.tpl.php

and not:

page--node--edit--mycontenttype.tpl.php

What is actually quite the same form?

Answer:
Benjamin: 2 weeks ago

Have you got the code in your template.php file set up correctly?

It should look like this:

function THEMENAME_theme($existing, $type, $theme, $path) {
    return array(
        'FORMNAME' => array(
           'arguments' => array('form' => NULL),
           'template' => 'page--node--add--mycontenttype.tpl.php',
          'render element' => 'form',
          ),

You can find FORMNAME, by viewing the source of the page and looking for the id of the form.