Probably because it's a lot harder to seize a site in another country which isn't technically breaking any laws.. It's different from say, megaupload in that no infringing content resides on their servers.
It's only going to get harder once TBP stops hosting even torrent files and moves to magnet links entirely. Once that happens, the site basically goes viral, since any random can just spider the whole site for magnet links and host a mirror.
Do you know the ease or possibility of scraping all the magnet links and creating a sort of Pirate bay on a flash drive? Because if so, they really live up to their motto.
It's only going to get harder once TBP stops hosting even torrent files and moves to magnet links entirely. Once that happens, the site basically goes viral, since any random can just spider the whole site for magnet links and host a mirror.