this message sent to me by google to know what's the reason :

* Make pages for users, not for search engines.
* Don't employ cloaking or sneaky redirects.
* Don't load pages with irrelevant words.
* Don't create multiple pages, subdomains, or domains with substantially duplicate content.
* Don't participate in link schemes designed to increase your site's ranking or PageRank.
* Avoid "doorway" pages created just for search engines, or other "cookie cutter" approaches such as affiliate programs with little or no original content.
* Avoid tricks intended to improve search engine rankings. A good rule of thumb is whether you'd feel comfortable explaining what you've done to a website that competes with you. Another useful test is to ask, "Does this help my users? Would I do this if search engines didn't exist?"
* If your site participates in an affiliate program, make sure that your site adds value. Provide unique and relevant content that gives users a reason to visit your site first.