Why do frames can't be indexed?

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I and my friends are using frame for our site http://extremecomments.com but we have been finding it so hard to be indexed by google. Although we are using nonframe tags to be indexed, still it's hardly be seen in the search engines for some keywords we used for the site. I am just wondering why is this so? You have a brief and clear explanation why frames are not being indexed by search engines. Please share thank you!