Some possible ones are the authors are not in the US or the statute of limitation on libel expired:
http://www.eff.org/bloggers/lg/faq-defamation.phpWhat's the statute of limitation on libel?
Most states have a statute of limitations on libel claims, after which point the plaintiff cannot sue over the statement. For example, in California, the one-year statute of limitations starts when the statement is first published to the public. In certain circumstances, such as when the defendant cannot be identified, a plaintiff can have more time to file a claim. Most courts have rejected claims that publishing online amounts to "continuous" publication, and start the statute of limitations ticking when the claimed defamation was first published.