Because the comments for different blogs are displayed in different styles, so they need different codes to crawl. Comments Parser contains specific descriptive information and codes to crawl the comments.
When you view an article in Google Reader, the extension will lookup the corresponding Parser. If the Parser exists, the extension will crawl the comments by the Parser.
If no suitable Parser, you can submit a request. Then you will be notified, if there is one suitable Parser. Of course, you can also create your own Parsers, and publish them to the web in order to share with the public.