Maintainability
A single change to the CSS stylesheet file can affect the entire site. Changes to site appearance can be cheaper, easier, faster -- thus more likely to happen. Sites are less likely to become stale. However, this also means that a single change can quickly 'break' the site's appearance, and hence designers must keep multiple backups of the CSS file.
The XHTML or HTML pages are primarily made up of content, so editing those files is less likely to 'break' the appearance of the site. Pages using CSS to store layout information are also easier to update with new content, because if the content is properly marked up, no new layout information needs to be added in order for the new content's appearance to be consistent with existing content.
Source: Wikipedia Style Sheet Development