Torrent sites are usually blocked due to the sites hosting copyrighted content or dangerous malware. Governments may order ISPs to block access to dangerous torrent sites, and search engines like Google hide links to torrent sites or demote them from the first few result pages to prevent illegal sharing of copyrighted materials. Some extensions, like Chrome’s AdBlock, filter out torrent site results from searches, for example.
In more severe cases, you’ll see a warning from a law enforcement agency that the site you’re looking for has been shut down. This happened when KickAssTorrents was taken down by the US government.