Jaap's comment is correct. It's unfortunately not possible to use a WYSIWYG editor on system templates because they're not (just) HTML, but contain Smarty code as well. Not only would Smarty break CKEditor, but CKEditor would break the Smarty code.
This unfortunately makes editing system templates difficult to do. However, one trick that makes it easier is to copy the text into software that understands Smarty templates. I use Netbeans IDE for this. While this may be overkill for most uses, Netbeans can recognize both HTML and Smarty tags and the tag that closes a section. So it's easy to use code folding to identify and hide the sections that don't concern you.
I work in Netbeans, then paste the text back into CiviCRM and press "Save". It's relatively quick to iterate this way.