Google's explanation was:
"As of April 30, 2007, we ceased providing the related links service. Related Links was one of many Google Labs products that we had developed experimentally as part of our commitment to ongoing innovation.
Through our evaluation of the Labs product, we identified the most compelling functionalities of Related Links and integrated them into new and improved products, like the AJAX Search API and AdSense Link Units. Current customers can now use these alternative products, which offer even more flexibility and are even more powerful at helping publishers make useful information accessible to their users."
Not many users were fond of the application as using AdSense could better achieve what Related links could.