The Control Panel has it's own UI (it's the only part of the package that isn't easily customization) so that if something happens in the development of your style you can easily navigate back to the style editor and manage things; if something happened in the development of your style and the admin area used it, you'd have a heck of a time navigating around in an attempt to fix any problems.
The admin UI's style is also used for the installer/upgrade utility, so if something happened to break your style you'd be unable to properly see elements in the Install/Upgrade utility as well as the Admin UI.
